I own like...one M rated game, haha. It was so exciting when I bought it, too. I finally got to use my ID for something and it was great. The game was Bioshock, and I personally think it deserves an M rating. If you've played it, you've seen what's in that game and that's probably too much for people in certain age groups.
Back on the subject of M ratings though, I think most of them suit their respective game. Like T!M said, there's not much swearing and gore in Halo 2 (as an example), but you are shooting and killing people/creatures. When you stop and think about killing in the scheme of things, that is a mature topic. I mean, I know most people don't think about it when playing a game, but I think that might have been the thought process to quite a few M rated games. The fact that you're killing and some people may have a hard time getting that across, especially a younger audience. We're taught that killing is bad.
That's just my perspective on it. I don't think that parents should be out buying their children M rated games personally, but parents are typically a good judge of their kids. They would know if their 13 year old is mature enough to play an M rated game or not. Even though there is a rating system, I think it's fine to use your judgement when buying children these kinds of games. I'd be the cruel parent that makes my kids wait til they're respective age on the rating systems though. I'm just mean and want them to suffer like I did. >:)