The reason to legalize it is because making it illegal does nothing to stop the use of the stuff anyway. You had to be 21 for booze and smokes in my day, and dope was a lot more illegal then. I don't think if it was legal we'd have used any more or less of it. And yeah, it's killed some friend early. But the money spent on law enforcement didn't help. It's kind of like gun control. That costs a lot, and doesn't reduce crime.
Forget about chemicals, advise against their use, it won't make a difference. If you don't want to be all messed up, don't do it maybe? Just like we do already.