people commit crime, it dosn't mean it should be legalised. A lot more people would start using it (which, lets face it, would mean smoking it) if the governent decriminalised it.
Regarding psycholoical aspects, I am not familier with evidence either way, but what I do know is this, it's addictive (and its combined with nicotine) and of the three people I know who smoked it regularly 1) friend from school, became very overweight (the 'munchies') and very paranoid 2) girl at college I knew, became very paranoid and needed psychiatric treatment for schizophrenia and 3) my cousin, serious user, hugely overweight. So okay, might be all correlation but having known these people I don't think so...
Secondly, and I think you'll agree that most people consume cannabis as joints, so there would be an increase of smoking related illness particularly so as one sucks harder on a joint and holds the smoke in longer than for a conventional cigarette.