Most people who are completely against legalization are the direct result of about 90 years of mis-information by the government. I don't like to use it myself but I rarely drink either. I just prefer living life without stimulants, I've always been that way. But I don't have a problem with other's using it, no more than drinking responsibly.
My main concern with complete legalization is over-use by young kids that use it as an escape from reality and do whatever is necessary to get more. I've seen it many times where I live, kids come to class stoned and it becomes like a cup of coffee, they can't do anything without having a joint before facing the world. Some stay that way until their late 20's (or longer) when they finally realize that living with their parents, and the inability to get and keep a job just "might" have something to do with being stoned all of the time.