My argument for legalizing marijuana, though I'm not a stoner, is that, when you legalize something which is illegal, the usage of it, in all probabilities, will decrease.
It's human psychology. If something is banned, people want to use it because of the adrenaline rush that comes from using something illegal.
If it's legalized, the "coolness" of using a banned item wears off and non-serious users won't bother with it.
If I'm not mistaken, in the Netherlands, a lot of drugs are legal. However, you never see the Netherlands topping the list of most stoners in a country, or anything like that, do ya? ;)