The modding system is useful for ranking comments so that when I read a massive thread the most interesting ones tend to be around the top.
It may also be useful for users as a feedback mechanism to figure out when they're doing something that's not cool (like the things we've been discussing in the guidelines thread today) but I don't think you need a precise numerical score for that. And I think it's the precise score that gets people in the karma-game mindset. It certainly does that for me.