Also, to respond to the OP.
MM, I realize that you partially remove ownership of your comments by saying "could be considered beneficial". Personally I think this is a cheap tactic -- a kind of trap in argument -- but I will play along with it. So I am not responding to you in criticizing this proposal; I am instead criticizing the proposal itself with no allegations of your ownership. If the proposal is not the same as your "potential" proposal, so be it; it is pretty clear what the proposal is regardless.
I think it is silly to say that alcohol and drugs could be put to the effect of self-development. Alcohol does improve your confidence, but this should not be taken seriously. There are plenty of things that improve one's confidence that aren't drugs themselves. For instance, having a hot girl accompany you to a dance probably improves your confidence. Or being the "alpha-male" among a group of guys probably improves your confidence in talking to a girl. But these aren't drug-related. So the implication that alcohol has some kind of therapeutic value in improving confidence is silly because therapeutic value implies that alcohol is the "best tool for the job", but clearly there are many "tools for the job" -- many ways of improving confidence -- and alcohol is one of the worst options, in light of its negative effects.
As for marijuana, it is silly to say that it could achieve mental growth, for it would be used as a crutch, and a crutch is always at odds with mental growth. Marijuana may not be physically addictive, but there would still be a dependence born out of this use of it. Not necessarily an addiction, but a dependence, and this would not be good; there would rather be a regression in mental growth. If marijuana really were such a great thing for mental growth, there would be an academic following to such a theory. The view that there is a conspiracy against people's happiness among academia and all of the various psychiatrical institutions is quite juvenile -- if marijuana really did make us better human beings, they would use it; they would not sacrifice the "right answer" and their reason to political bias.
Personally, I think marijuana should be legalized, but not because it has value for mental growth. I think it should be legalized simply for the reason in that people who smoke it are getting ripped off in terms of price, and also because it is far less safe getting it on the black market than it would be from a convenience store. I also don't think it's very harmful -- much less so than cigarettes -- so I think it makes sense to legalize it. On the contrary, I think it is quite silly to exalt marijuana, and I think that part of the reason why it is so glorified is for the very reason that it is illegal. A certain demographic that argues for its legalization has a vested interest in its illegality, the existence of which they are probably naive to.
I think that in general the glorification of alcohol and drugs is very naive and selfish. It is glorified simply because people want to find spiritual value in it; the desire to find this value precedes and anticipates the actual discovery of the value. I think it would make sense that such a trend exists in the atheist population, as they feel keenly the absence of spirituality in their choice to desert or abstain from religion. But spirituality can be found in other, better ways, in more honest ways, and I think it is silly to turn to drugs and alcohol as a gateway to spirituality. It is a rather desperate, naive, and self-serving choice -- it is also somewhat self-deluding.