The people that really want to use weed, do so already. Making it legal might cause more people to use it initially, but I don't think that trend will stay that way. Lots of people try it a few times and stop using.
Also, if its legal, it'll take the fun away for those using it who are trying to be rebellious. Just like its illegal to drink in the US 'til you're 21. A lot of the people I know drink pre-21, and upon hitting 21, they go nuts buying alcohol 'cause they can, and then after awhile slow down.