The point is not so much to stem the use as to eliminate the criminal aspect of it. When was the last time you heard of any large-scale cartel smuggling gasoline or alcohol into the US? There just isn't enough profit in them to justify it. And when you eliminate the criminal charges for buying or selling LEGAL drugs, you tend to limit the possible clients for those selling illegal drugs. Just too much risk involved to justify the small profit.
Personally, I would cast a wary eye on the motives of those who would oppose the legalization of drugs. The possibility that it would cost them a lot of money can't be overlooked.