I'm surprised most people on this thread wouldn't even do it out of curiousity. Weed is non-addictive and having a few tokes is about as harmful as having a couple of bottles of beer.
Basically, if you try it, you either decide you don't like it and don't do it again (but now from a point of experience rather than naivety), or you decide its actually fairly pleasant and may do it again from time to time.
Provided you don't become a stoner (and its pretty difficult to get addicted to cannabis from wht I've heard), there is no problem with doing it occaisionally. Its less harmful than alchohol. Furthermore, drunk people can get loud, violent and stupid, which I think is far worse than being mellow. Alchohol not only causes more damage to yourself, but also to other people (drunk driving, getting into fights etc.). I don't see how you can drink alchohol but be against cannabis and have a consistent argument.