To be honest, I think that it should be legal. The individual that chooses to use marijuana does so according to his own free will. Though the government has the right to limit those actions if they pose any significant threat to the individual, however, marijuana is far less dangerous than most drugs, which are legal, such as alcohol.
Another reason is that keeping marijuana illegal is expensive for the government, be it federal or local authorities. Making it legal would save them a lot of money, instead of spending them chasing peaceful people who happen to like to get 'high'. Oh, and they also can force taxes on marijuana users, and can pay for programs intended as a effective drug education.
There are other reasons, like prohibition actually increases drug use, but my hand is hurting me, so..