I've been managing and running forums for almost 20 years now, and to this day, I still do not understand the point of post ratings [on forums]. They came about when Dig was hip, and it was used in that system to bump topics in the list (more digs, higher it's ranking). Forums don't work like that so there is no real point.
I did like the later "Thanks" system, where a lot of forums focused on help/support and this allowed a user to be thanked for their input by people that doesn't need to add input and clutter the post just to say thanks.