If it's legal to go and get a fifth of whisky and a carton of marlboros i can't see any real reason to ban any substance. I never really understood why it was illegal in the first place. It should be regulated and taxed, and taken out of the black market. I'm not sure how much money is involved in the marijuana trade, but i'm sure it would generate a great deal of tax revenue if it was legal. This money now goes to criminal elements.
In any case, criminilization clearly hasn't been very effective in stopping recreational use. Many who use it recreationally are otherwise entirely law-abiding. Any antisocial behavior that it could theoretically lead to is already illegal. It just makes very little sense to me.