"Nothing worse could happen to these labours than that anyone should make the unexpected discovery that there neither is, nor can be, any a priori knowledge at all.... This would be the same thing as if one sought to prove by reason that there is no reason" (Critique of Practical Reason, Introduction).
You don't need Kant to demonstrate the value of honesty. In fact, summoning his revenant can be a dangerous thing to do. You end up in the somewhat undesirable situation of having almost the right conclusion, but having it for the wrong reasons. Reasons you weren't even aware of, because they were all collapsed into the belief, "I believe in person X".
One of the annoying things about philosophy is that the dead simply don't die. Once a philosopher or philosophical doctrine gains some celebrity in the community, it's very difficult to convince anyone afterward that said philosopher or doctrine was flawed. In other words, the philosophical community tends to have problems with relinquishment. Therefore, there are still many philosophers that spend their careers studying, for example, Plato, apparently not with the intent to determine what parts of what Plato wrote are correct or still applicable, but rather with the intent to defend Plato from criticism. To prove Plato was right.
Since the community doesn't value relinquishment, the cost of writing a flawed criticism is very low. Therefore, journals are glutted with so-called "negative results": "Kant was wrong", "Hegel was wrong", etc. No one seriously believes otherwise, but writing positive philosophical results is hard, and not writing at all isn't a viable career option for a professional philosopher.
To its credit, MBlume refrains from bringing up Kant in his article on radical honesty, where he cites other, more feasible variants of radical honesty. However, in the comments, Kant rears his ugly head.
"Kant disagrees and seems to warn that the principle of truth telling is universal; you can't go around deciding who has a right to truth and who does not. Furthermore, he suggests that your lie could have terrible unforeseen consequences.
I am more utilitarian than Kant, but it is not hard to ignore "proximity" and come up with a cost/benefit calculation that agrees with him."
"Is this question really so hard? Remind me never to hide from Nazis at your house!
First off, Kant's philosophy was criticized on exactly these grounds, i.e. that by his system, when the authorities come to your door to look for a friend you're harboring, you should turn him in. I briefly scanned for clever Kant references (e.g. "introduce the brownshirts to your strangely-named cat, Egorial Imperative") but found none. Kant clarified that he did not think it immoral to lie to authorities looking to execute your friend."
The problem with bringing up Kant here is that he simply doesn't belong. "Don’t [lie] to anyone unless you’d also slash their tires, because they’re Nazis or whatever," is very different from Kant saying (paraphrasing), "Never lie, ever, or else you're a bad person." An argument against the former by conflating it with the latter doesn't accomplish anything. Further, there's no mention of all the stuff Kant has to assume in order to argue for the Categorical Imperative and, finally, the value of radical honesty.
Luckily, we only need the first couple pages of the Critique of Practical Reason to get to the Categorical Imperative. I want to flag down three very large assumptions that Kant needs, which I believe few rationalists would want to espouse. First, let me fill in the latter part of the inferential chain: given the existence of freedom, God, the immortality of the soul, and a supernatural consciousness, Kant will argue that any mind with a "morally determined willpower" will conclude that it should act in accordance with subjective principles that in principle could be universally applicable (i.e., the Categorical Imperative). I don't want to get in to what that actually means for Kant, as it's not really relevant, but suffice it to say that the Categorical Imperative implies that lying is always, anywhere, and for anyone ethically wrong.
Freedom, God, and the Immortality of the Soul
Skip this section if you don't care about Kant.
Freedom here means completely acausal, metaphysical freedom from a Mind Projection Fallacy that treats our mind as somehow different from the body. Kant uses the concept of metaphysical freedom (and not, for example, merely our everyday experience of determining our course of action) to argue that there are such things as moral laws.
"Inasmuch as the reality of the concept of freedom is proved by an apodeictic law of practical reason, it is the keystone of the whole system of pure reason, even the speculative, and all other concepts (those of God and immortality) which, as being mere ideas, remain in it unsupported, now attach themselves to this concept, and by it obtain consistence and objective reality; that is to say, their possibility is proved by the fact that
freedom actually exists, for this idea is revealed by the moral law." (CoPrR, Introduction)
I think in a perverse way Kant knew he was becoming Escher-headed by believing in metaphysical freedom.
"Lest any one should imagine that he finds an inconsistency here when I call freedom the condition of the moral law, and hereafter maintain in the treatise itself that the moral law is the condition under which we can first become conscious of freedom, I will merely remark that freedom is the ratio essendi of the moral law, while the moral law is the ratio cognoscendi of freedom." (CoPrR, Introduction)
If one doesn't assume completely acausal, metaphysical freedom and tries to follow Kant's argument, the whole thing falls apart. There's no longer (for Kant) any reason to believe in moral laws, and therefore in the Categorical Imperative, and therefore in radical honesty.
God here is, strangely enough, not necessarily the Christian God, though presumably Kant meant as such. Both it and an eternal soul are necessary to realize the goodness of the Categorical Imperative described above. Without either of these, there's no reason to obey the Categorical Imperative, as being "Good" would then simply be impossible.
"The realization of the summum bonum [the Greatest Good] in the world is the necessary object of a will determinable by the moral law. But in this will the perfect accordance of the mind with the moral law is the supreme condition of the summum bonum. This then must be possible, as well as its object, since it is contained in the command to promote the latter. Now, the perfect accordance of the will with the moral law is holiness, a perfection of which no rational being of the sensible world is capable at any moment of his existence. Since, nevertheless, it is required as practically necessary, it can only be found in a progress in infinitum towards that perfect accordance, and on the principles of pure practical reason it is necessary to assume such a practical progress as the real object of our will." (CoPrR, Chapter Two, Part IV)
Moral of the Story
What we have then is a very powerful theme that has woven its way into our list of cached thoughts. Whenever someone mentions the value of being honest, some proportion of the population is primed to think of Kant and his variant of radical honesty to the exclusion of other variants. Some proportion of that proportion is then primed with various anti-philosophy memes which immediately attack Kantian radical honesty to the conflation of it with other things. What is lost is the realization that Kantian radical honesty is in this era a straw man; everyone already knows it (and attempts to fix it while still being authentic to Kant, i.e., Kantian Studies) is inherently flawed, because it is based on a set of irrational assumptions.
My suggested strategy to avoid this in the future is this: whenever you find yourself citing the beliefs of another person, try to avoid referring to them as "the beliefs of X" unless you are actually talking about their beliefs (or the beliefs recorded in their writings, etc.). Be aware of creating straw men by comparing your interlocutor's beliefs with the beliefs of a famous philosopher, and certainly don't knock your straw man down by citing the beliefs of one of that philosopher's critics.
EDIT: Made it more obvious that MBlume proposed more than one variant of radical honesty.