From a great number of people's reports about their experiences while using psychedelics, they all seem to follow a common theme (things about opening their mind to God, communing with the great underlying themes of life, opening previously locked doors in their minds). If a plurality of people report having the same experience, then to me, that validates it.
Then there's the solipsist point of view, which is to say, it doesn't matter that someone else thinks you are deluding yourself, because all that is knowable is in your own mind, so you aren't capable of validating anything anyway.
Either way, the experience is validated, to one's self or to the world.
The experience which is validated by many independent tests is the feeling of tapping into something truer and deeper. I don't think the GP is questioning the feeling, just the actual truer, deeperness of it.
The question that scientists and other empiricists ask when considering ideas is not "is it true?" but "is it useful?"
Do people bring back useful (in the engineering sense) knowledge from their times tripping/rolling? Are they more likely to experience e.g., fruitful insights akin to Kekulé's vision of the benzene ring as an ouroboros, as a result of having consumed these drugs? If so, then the drugs may indeed live up to the claims that they grant access to "truer and deeper" levels of reality. Otherwise, their users are simply fooling themselves. (Although fooling yourself can prove useful; the almost universal belief in deities among humans fulfills some sort of purpose, though it is my belief that that purpose is no longer relevant.)
You're using the word 'useful' as a euphemism for 'practical,' and missing the point. You cannot expect to fully grasp a wholly subjective experience through a predominantly rational mindset. Better to simply discard the intellectual point of view for this topic and attempt to engage it otherwise.
Then there's the solipsist point of view, which is to say, it doesn't matter that someone else thinks you are deluding yourself, because all that is knowable is in your own mind, so you aren't capable of validating anything anyway.
Either way, the experience is validated, to one's self or to the world.