I don't rate the books here with any kind of regularity, and when I do it tends to be sort of an impulse thing (always a 10) based on the sheer joy of discovering that the book is here (and particularly if it's a crisp-looking individual copy of the book, or the scan itself is exceptionally clear).
Recently I had some sour comments on the (lack of) quality of the writing in the last few issues of Planet Comics, but I never even thought to give those books a 1 or 2 rating, I just left comments explaining why I thought the stories were crap. By no means does that imply that I'm giving the uploader of those books a backhanded slap for their efforts, because if the issues weren't uploaded on here, I wouldn't have had the opportunity to read the stories and judge them on their merit (or sad lack of same). By the same token, I've left some "thank you" comments on the book directed at the uploader of the comics when I felt grateful.
It's a good question to ask, though -- What is it exactly that's being rated? It could be a good-quality scan of a very hard to find book, but if the contents of said book are crap, that's not the scanner's fault, nor should they take a negative rating as aimed at them personally.
When all's said and done, if you want to give a big 'thank you' or 'attaboy' to the uploader, it might be better to try to remember to click on the member's name and give some applause (or karma).