Beauty is in the eye of the beholder...It's difficult to please everyone.
To just give a poor or high rating because of reasons other than the performance or aesthetics of the skin itself..well that's not cool...unfortuanately there are a few out there that would do that.
On the other hand, If 10 people rate a skin 10 then someone thinks the skin is just average and gives it a 5, I don't see a problem with that.
As far as ratings go...There are really very few that rate...I have one skin rated 8 with over 20,000 downloads and only 8 people rated it
For me it's not too big a deal, but that's just my take on it