There are always two sides of every argument. I don't see why it should be illegal when cigarettes aren't, nor alcohol. People just need to be educated on the risks of what the put in their body before they do something stupid e.g. Killing someone in a car accident because you were high/drunk.
I'm not saying legalise it, I'm heading more towards... there is tonnes of other shit that isn't illegal that is just as, if not worse, for you. If you're gonna do it, keep it to yourself and leave other people out of it.
Be responsible.