I've never liked rating systems on forums. They can be so misleading. Some of the people rate a post because it's good/bad, some because they agree/disagree with the post regardless of its merits and some because they have a personal like/dislike of the original poster. So it ends up being meaningless. I don't need other people to make up my mind for me (at least, not most of the time - lol!)
However a post rating system is not nearly as bad as a poster rating system, which can be extremely damaging to a forum.
