I think the rating of videos is not used as much as it should and many times, even though it is used, it's not helpful. Let me explain:
the fact that most videos only get a couple of ratings is not news to anybody, I think. So when you look for a video, you see it has 5 stars and perhaps you are tempted to download it, but in fact the video has 1, 2, maybe 3 ratings … so it's almost useless.
on the other hand, sometimes you see videos that have a rating of 2 stars (or worse), but you don't know why, because the snapshots look good. It gets even funnier when the same video (uploaded in different formats) has different ratings, even though it's THE SAME MOVIE.
I think we should all start to rate a video after we see it, but at the same time, leave a post on the video explaining why we liked it or not. For example you give a video 2 stars and then you leave a post, saying something like "I only liked the first two scenes", or "many guys have limp dicks", or "the bottoms can't really take it" ... stuff like that.
And while some uploaders may feel offended by comments like that, everybody is entitled to an opinion and besides that, the other people will be able to use their ratio and bandwidth downloading better movies. :)