Thanks for reading and for this very reasonable and relevant question! My overall primary concern is, indeed, (ethical/effectively altruistic) anti-natalism in general. I do think, though, that the specific justification for reproducing life which is that we need brains in order to do intelligence augmentation/amplification research on is especially (frankly, no undue offense intended) insane. I'm posting this essay on Less Wrong because Yudkowsky maintains an underlying transhumanist/cryogenics/intelligence augmentation/amplification presupposition that I think is, firstly, popular with members of this forum, and, more importantly, demonstrably irrational (despite Eliezer's being an obvious badass rationalist in so many other areas of his thought--AI-misalignment being the big one). That is, this (childbirth = good because intelligence augmentation = good, and reproduction means more--raw, at least--intelligence) is a particularly illustrative example of optimism-bias (all cognitive biases are members of the set "optimism bias", in that they all assume a false positive--hence my "pessimism" is, still, as I see it, a "realism"). So yeah, antinatalism in general is my main pragmatic message (provided "antinatalism" includes not only no-reproduction but also prioritizing caring for existing children above all else, including the issue of no-reproduction, even), and I'm drawing attention to this specific example of pro-natalist/vitalist thinking, here on Less Wrong, because I think this is a useful introduction to the overall issue of stop-reproducing-for-g-d's-sake, and because I feel this is the community most likely to take this seriously and examine the cognitive bias at play here. Hopefully that might include Yudkowsky himself, but I'm grateful to have conversations about this with anyone capable of approaching it rationally, which includes you, so, again, thank you!

Thanks for sharing your thoughts, Mei. 

I’m realizing it might be helpful for me to start compiling the main responses I receive to ethical anti-natalism as I’ve summarized it above, to make it easier to identify essential points of disagreement. 

Firstly, my argument in summary (effectively altruistic anti-natalism; EAAN)—the first point I forgot to address in the original essay (or took for granted): 

• (1) Bodily autonomy: in general, the individual body is an inviolable domain, the sole property of every individual. Should we stop a child from walking into the street in front of an on-coming bus, or from playfully tipping over the pot of boiling water on the stove onto themselves, and so on? Sure (that is, yes, for crying out loud). But do we have ethical grounds for preventing a mother from choosing an abortion? No. Thus, neither is there any ethical grounds for intervening into free-choice reproduction. The choice to reproduce or not is inviolable. 
• (2) Reproduction = (a) non-defensive (b) +suffering, (c) +death, (d) w/o consent: however, reproduction invariably and non-defensively increases suffering and death without consent. It is unethical, therefore, and any justification for reproduction that invokes the idea of decreasing suffering and death is nonsensical (and abusive, more importantly). Childbirth should be discouraged, just as abortion should, in principle, be avoided/discouraged (in the form of availability of birth control tools and singing the praises of celibacy and non-reproductive sex), even though there is no justification for actively intervening into either (due to bodily autonomy). 
• (3) Existing children are completely innocent, and utterly helpless/dependent. Attending to the well-being of existing children is the top ethical priority for humanity, whether we acknowledge this or not—it is the most “fundamental question of philosophy” (not suicide, as Camus claims). Therefore, disregarding or abusing existing children in favor of the non-existent “unborn” or non-existent “future children” is unethical, delusional. The top priorities of EAAN: advocating for existing children in every conceivable way; advocating for the CAVE (Compassionate, Accessible, Voluntary Euthanasia); discouraging reproduction on ethical grounds. “Eugenics” is not EAAN, whatsoever, nor is encouraging people to kill themselves, nor is denying people the dignity of non-abusive joy. “Abusive joy”, importantly, is also no justification whatsoever for abuse. “A mother’s joy” is still a form of abusive-joy, fundamentally, regardless of the good intentions paving the road-to-hell which is the birth canal.  

EAAN is fundamentally anti-abuse. The Basic Seven Abuses/Moral Catastrophes of EAAN: 

• Survival-At-All-Costs (killing and eating the sick cabin boy to survive being stranded at sea, or participating in genocide with a gun to your head only to avoid being killed yourself—the correct answer is to accept starvation, not murdering and cannibalizing the cabin boy, and to accept being killed rather than participating in genocide. See: “Custom of the Sea”, and, you know, every genocide ever.)
• Childbirth (extreme limit of child abuse)
• Child Abuse (extending from birth)
• Adult/On-Going Abuse (extending from child abuse)
• Slavery (extreme abuse)
• Sexual Assault (extreme abuse) 
• Genocide/Murder (combines all of these horrors into a horror-of-horrors, basically a divergent infinity of horror)

The usual serious-minded negative responses to EAAN:

• “Naturalist Defense” (ND) / “Survival Instinct” (SI): Nature is life. We reproduce because we’re designed to reproduce. It’s just what we do. To deny humans this fundamental instinct would be to deny them their humanity, their own nature. We seek life because we are life. We all want to go on living, we all have the ‘survival instinct’. To deny the survival instinct would be tantamount to murder (or is at least leaning in the direction of indifference to murder). Anti-natalism is the mere celebration of death at the expense of being grateful for life, as we’re designed to be by nature (survival instinct = gratitude for life = life is a gift). Refutation: to start, to paraphrase Jean Amery, “we don’t actually have to live, and, news flash, we’re going to die” (suicide’s a thing, death’s a thing). Secondly, if the reproductive and survival instincts are so inviolable according to Natural Law, why are there anti-natalists? We’re a minority, sure, but a non-zero group, throughout history. ND/SI is no different than presuppositionalist fideists (see: von Til) claiming everyone actually believes in God whether they admit it or not despite the obvious fact that many people don’t (“Why did God create me to be an atheist, then?” / “Why did Nature design me to be an anti-natalist?”). This is religious/magical thinking, in other words. Nature is not “pro-life”, but life-tolerant and ultimately pro-death. Thirdly, if the reproductive/survival instinct is an inviolable Natural Law, and if we suppose that everyone is actually deep-down pro-natalist by natural default, why do all humans not reproduce as often as possible until they physically can’t (you know, the way mothers have historically been forced to do against their will in many societies throughout history into the present, and the way many other animals do)? Would such militant pro-natalism be a good thing? Few ND/SI advocates are willing to adopt this (insane) position (even many contemporary religious pro-life advocates don’t go this far—even many/most hyper-conservative theonomists don’t go this far). Yet, this is the logical conclusion of ND/SI. More reproduction = more life = more better. Except why does it make everything worse, then? (Reproduction = increasing suffering and death and lowering the already very low likelihood of attending to existing children, i.e. it makes everything worse.)
• Joy > Suffering (J>S): Joy is worth suffering for, worth living for. One moment of ecstatic joy may be worth a lifetime of otherwise terrible pain. Primo Levi was an optimist, right? No matter what might happen to our children, we have reason to believe they can experience ecstatic, life-affirming joy. Therefore, having children, if not mandatory [see ND/SI] is still always justifiable. We can and should weight “joy” conceptually to be worth far more than suffering, or at least consider the joy/suffering ratio to be 1:1, 50/50. Refutation: firstly, Primo Levi, let’s face it, committed suicide, same as Jean Amery (Amery left an “On Suicide” essay and a suicide note that were consistent with all of his other writings, though). Denying this is an example of the same optimism-bias that Levi was (in)famous for. In other words, even this person appears to have considered life not-worth-living at some point. More importantly and regardless, while we EAAN-ists are a minority, sure, there are still countless examples of people like myself who either openly consider life to be not worth living (even if, like me, we consider going-on-living to be worthwhile only to try to defend innocent children from abuse as we’re able), who regret having been born (and thus resent having been left out of the decision-making process by default), or who take their own lives. To paraphrase Heine: “the living are to be pitied, the dead are to be envied, and best is never having been”. There is some wisdom to the idea of “don’t make the best the enemy of the good/better”, but only in scenarios where “the worst” is highly probable/avoidable only with great difficulty, and where “the best” is highly improbable/difficult to obtain in comparison with a much more probable “good/better”. Childbirth is not one of these scenarios. It’s actually quite easy to choose to not reproduce, and thereby achieve “the best”. It’s a two-step looping algorithm: (1) don’t reproduce; (2) repeat. Sure, I’m mostly asexual (no surprise, I’m sure), so this is easier for me than it is for others. But it’s still comparatively quite easy. It’s infinitely easier than any “alignment” project, or any utopian project whatsoever, that’s for damned sure. It’s also 100% effective, at next to zero cost, excepting the probability of being forced to reproduce by others. Lastly, objectively speaking, there is more existential suffering than there is joy. The larger the existential dataset we consider, the more we are forced to conclude that existence is subjectively non-dialectically unilaterally negative, qualitatively (“life is suffering” is, sure, somewhat of an overstatement—it’s also relief, joy, ecstasy, bliss, good-times of whatever kind—but it’s also mostly true). If we forego artificially weighting either suffering or joy in a biased manner, and consider both qualitative categories objectively, suffering dominates joy by a landslide. In other words, “survival instinct” = death-denial/death-anxiety/fear-of-death, not “life = gift”. We don’t need to acknowledge this fact to conclude EAAN. But, if we’re going to go down the road of objectively calculating suffering vs. joy, suffering dominates. We all know this. We only deny it due to bias (death-denial/death-anxiety/optimism-bias/sunk-cost fallacy/etc.). The simplest illustration of this I can think of is the classic “glass half full/half empty” thought experiment. Like all ill-formed thought experiments, this one assumes its conclusion without actually demonstrating it in the experiment. We’re supposed to conclude that this ½-full/empty glass is a neutral object, interpretable however we wish. But the reality is that, if you assume the “optimistic” interpretation, you can’t have your water and drink it too. The utility of the water is in drinking it—the taste of water is thirst, as I like to say. That is, great, ½-full. Now what? You have to drink the water. And then your glass is empty. You need more water. Do you have more water? You can’t just optimistically assume you do, can you? Nope, you have to actually go find more water. And there is no guarantee of even a ½-glass of potable water unless you already know you have access to it. And that’s the optimistic interpretation. The realist/pessimist interpretation is to simply point out that, even if you decide to have your water rather than drinking it (which means you’re defining dying of thirst to be “good”, which is already an obviously pessimistic conclusion), if the amount of water is, say, ~150ml, the glass will be empty due to evaporation in something like ~20 days, evaporating at something like ~.0001ml/sec (or something like that). That’s the universe we live in. The glass, eventually, is empty, like it or not. Is dying of thirst worth savoring one droplet of water? (No. It isn’t. Childbirth, in this universe, is unjustifiable.)   
• The Unborn Deserve Birth (UDB): Who are we to deny the unborn the joy of living? Refutation: the “unborn”, or future-children, don’t exist. This idea of the unborn deserving life is a reification, a pure conceptual abstraction, a fiction. There are no such “children”. “Children” means existing children. (Sorry for the repetitiveness, but once you see this fact for what it is, it’s mind-boggling that anyone can’t see it.) We cannot commit abuse against the non-existent, whatsoever, ontologically, but ethically we certainly can’t justify abuse against the existent on the grounds that this avoids abuse against the non-existent. This idea is manifestly nonsensical. If there are 100 children in need of scarce resources, creating infinitely many more children to distract from these initial 100 children is abuse committed against not only the initial 100, but against this entire infinite set of children. If you’re struggling to finish 5 tasks at work by the end of the work-day, knowing you’re very unlikely to finish but trying your best, and your boss comes by and drops another 12 tasks on you and says they need them all done by the end of the day, then walks away, all your boss has done is layered failure onto your already likely failure (to attend to even 5 let alone 12 of these tasks). The 12 new tasks can’t take any real priority over the initial 5, because you weren’t even going to finish the 5 without a mad rush of effort. Your only hope is to, by the end of the day, discover/invent an entirely new revolutionary method of task-completion (good luck). The boss has simply made everything worse, and is accepting no responsibility for this—they are projecting their (ir)responsibility onto you; you’re to blame, the boss assumes. Creating more children is invariably abuse against existing children, including/especially those most recently born—it only lowers the likelihood of attending to the needs of existing children, invariably.   
• The Unborn Have No Rights (UHNR): Sure, we can’t obtain consent from the non-existent, but because this is impossible, we are justified in reproducing without consent. Pretending that the unborn have a right to not-exist is nonsensical, since they have no rights. Therefore, bringing a child into existence can’t be considered a violation of consent, and is thus always or at least mostly justifiable. Refutation: firstly, this idea goes entirely against UDB above. The unborn can’t both have no rights and deserve to exist, this is nonsensical. Which is it? Secondly, it’s wrong to sexually assault the unconscious. Right? The inability to obtain consent does not justify committing abuse without consent. The non-existent have no rights, sure—because they have no anything, they don’t exist. But this, of course, doesn’t carry over into existing. The existent have rights, including the right to not being non-defensively submitted to suffering and death without consent (which has been violated as of their existing). Hence, childbirth is abuse against the existent, by definition, not the non-existent (against whom there can be no abuse, by definition).    

All the other usual negative responses to EAAN in my experience amount to mere hysteria, an emotionally understandable but still irrational freakout in the face of an initially disturbing (yet irrefutable) argument—they’re not worth mentioning here, that is, as I see it. 

Hope this helps (prevent childbirth/promote lessening child abuse).