It would be helpful when you write a post like this to explain the concept of what a subordinate tense happens to be in Spanish for those people who don't know Spanish.
The subjunctive mood and really anything involving modality is complicated. Paul Portner has a book on mood which is probably a good overview if you're willing to get technical. Right now I think of moods as expressing presuppositions on the set of possible worlds you quantify over in a clause. I don't think it's often a good idea to try to get people to speak a native language in a way incompatible with the language as they acquired it in childhood; it adds extra cognitive load and probably doesn't affect how people reason (the exception being giving them new words and categories, which I think can clearly help reasoning in some circumstances).
The subjunctive tense is (if I remember correctly) used in all subordinate clauses. For example, look at the phrase “Es 50% probable que el libro sea rojo,” translating to “(It) is 50% probably that the book is red.” “Sea” is the present subjunctive form of “ser,” which means “to be.”
I used google translate on this, but as it is a simple sentence it should be accurate. I only have a few years of Spanish education so I’m not fluent or anything, but I am fairly confident this example holds.
Your example is right, but it's not true that it's used in all subordinate clauses. For example, "Estoy buscando a la persona que escribió ese libro" (I'm looking for the person who wrote that book) does not have any verb in subjunctive mood.
My perspective as a native speaker who doesn't remember his grammar lessons very well:
The subjunctive mood has a lot of uses, at least in Spain (I'm not really familiar with other varieties of Spanish). Some examples off the top of my head:
1. Counterfactual conditionals: "Si Lee Harvey Oswald no hubiera disparado a JFK, alguien más lo habría hecho" (If Lee Harvey Oswald hadn't shot JFK, someone else would have), here "no hubiera disparado" is subjunctive and means "hadn't shot".
2. To speak about people's actions or decisions which depend on preferences. "Hará lo que quiera con el dinero" (He'll do what he wants with the money), here "quiera" is the present subjunctive of "querer", meaning "to want".
3. To speak about properties of unknown entities. "Quien pueda trabajar será pagado" (Those who can work will be payed), here "pueda" is the present subjunctive form of "poder", which means "to be able to".
Here is a fairly comprehensive list of uses (in Spanish 😉)
I think in general the subjunctive mood conveys some degree of unrealness or subjectivity. You could probably say many of the examples above using indicative mood only, but you would definitely lose some expressive power (I don't know why this is not the case in other languages)
I remember being super confused when I was learning English because of the lack of a distinct subjunctive verbal form. Say, in "I wish I had had a car back then", the two "had" have completely different meanings, one for past tense and one for expressing desire. The Spanish equivalent would be "habido" and "hubiera" from the verb "haber" respectively.
Do any Spanish speaking rationalists use subjunctive tenses? I've had the sense for a while that it's necessary for when you're expressing unfocused uncertainty or a strong emotional attachment to a belief or opinion, but that most of the time you can get by just fine with indicative tenses only. You can express uncertainty as a factual statement about how certain you are of a claim's accuracy. You can express strong emotions by making a factual statement about how you feel about something.
Then again, maybe it takes more words to do it that way. But surely emotions can be gleaned well enough from context and tone without using subjunctive tenses. Other languages such as English don't have any subjunctive tenses at all and that doesn't seem to cause any communication difficulties. (Although I could be wrong since English is my first langauge.)
Rationalists have better tools for focusing uncertainty and being more aware of one's emotions and learning to avoid investing one's emotions in their beliefs too much because there's always a chance they might be wrong. Basically I'm wondering if that causes Spanish speaking rationalists to reduce or eliminate their use of subjunctive tenses.
Thoughts?
Edit: someone pointed out that English does have subjunctive tenses and I just didn't notice it. Now I'm wondering if the use of subjunctive tenses is less clear cut than a quick google search can explain. I thought it was just for expressing emotions and uncertainty (which you can technically still do in indicative). But it might also be necessary for politeness or speaking courteously to others. ("Would you like something to drink?" is subjunctive right? And "Do you want something to drink" is indicative?)