I've seen many of my friends waste years of their college lives running around like stoned hippies, overeating junk food, and watching absolute trash with loud sounds and bright colors. To make things worse, it's considered a great "social activity", much like going to a bar to get sloshed with friends. Only weed is _much_ easier to get hooked onto, because it's much cheaper than alcohol, and you don't need much more than rolling paper (as opposed to refrigerator, soft drinks etc).
That said, many of them have had very eye-opening experiences where they learn a lot more about the world and themselves. They have formed strong social bonds on the basis of meaningful experiences, and have learnt moderation. They've also found it to be an extremely good analgesic and laxative. Ofcourse, there are sections of society that need it more than anything: chemotherapy patients take it for increased appetite, and people with chronic headaches take it to reduce occurrences (although LSD has been shown to work with an even more extreme form: cluster headaches).
Every substance in the world has a potential for abuse: there are plenty of "sugar junkies" who will live horrible lives and die a painful death, for instance. All in all, the issue is that of education: we need to legalize all these substances, and objectively explain how people can practice moderation. In general, enforcing hard-abstinence is a very poor solution; only repressive regimes should practice them.