Everything is good for you in moderation. Drugs and alcohol carry negative connotations because of their high potential to be abused, often times by those who are not emotionally stable or in the best situations. It's like an obese person eating himself to death but overeating was never a crime. It is a slippery slope because casual recreational use of either can lead to a dangerous and destructive relationship, but not in every case. You just have to know if you have an addictive personality and regulate yourself. That's not to say we can all be trusted to regulate ourselves...but the argument is the same with alcohol and what is the appropriate legal age should be.
I don't think drugs are bad. I think that dependency on it becomes very dangerous. As someone who appreciates art, I understand that many artists use drugs recreationally and have created some of their best work while under the influence. Jean Cocteau, Charles Baudelaire, Aldous Huxley, Jack Kerouac, Coleridge...just to name a few.
The world has benefited immensely from works such as Dr. Jekyll and Mr. Hyde, On the Road, and Requiem for a Dream.
In art it helps us achieve a higher altered state. In health, it has helped numerous suffering from debilitating pain.
Everything is good for you in moderation. But once you go down the rabbit hole, you may never come back and some are not prepared for that sort of free fall.