I don't really see a need for it. I'm on another forum with such a system, and the only use for it I've seen is a way of saying "thanks"... but you can do that via replying to a thread or PM as well. So the only case where it's really superior is if a thread is old and you want to say thanks, but don't want to necro... and PM'ing is still an option for that even without a rep system.
The reputation points is kind of pointless. Once you're at a forum for long enough, you figure out who the movers and the shakers are anyway, but it's less like a popularity contest without "reputation" being displayed. So, having been here awhile, I know for the most part who the great mod makers for Civ3 are (and some of them for Civ4), who the top-level players are, who's known for their stories, who the moderators are, etc. As well as who tends to be able to offer good advice in certain areas, such as unit making or historical accuracy. The only thing a reputation number would give is an indication that someone is known for something, but with no indication of what.