I realize the example I used in class of the farmer whose vocation it was to grow weed was a crappy one. So I’ll take another stab at making a case against “Tom’s” lifestyle choice. Yes, one could argue that if the Beatles hadn’t of done drugs, then they would never have created their most original work. On the other hand, one could also argue that Charlie Parker wouldn’t have ruined himself and died prematurely if he hadn’t gotten messed up on drugs. Same could be said for Kurt Cobain. In both these cases, surely the harm principle would say that had both these figures lived longer, society would have benefited.
Anyway, I’m not saying that there should be a complete ban on drugs, or that drug use should be encouraged, but rather that middle-ground regulation is probably the most justifiable. This way we acknowledge a distinction between drug use and drug abuse.