Still, one can ask if generally speaking, a person is better off learning Skill X.

Doesn't stop one from answering that, generally speaking, it depends on the person and circumstances. :-p

On a more serious note, I think that it is rather different to ask if for a skill X, X is more useful than not to the sort of people that learned X, as compared to asking if a random person would benefit from X. For example, I'd say that learning neurosurgery procedures is useful to a huge percentage of the people who learned it, but useless to the average person. I'd say rationality skills are probably most useful to precisely the sort of people who would not learn any, while providing diminishing returns to rationalists