On the other forum I use there is similar system, where you downvote spam and useless, off-topic posts and upvote quality ones. Points resets every 24h, and amount of those points to vote depends on the forum rank of user. It's not a bad system, but people tend to give negative points even to good posts just because they disagree with poster. On the other hand, because points are renewed every day, there is no reason to hold them.