Prolific book reader and insightful book reviewer Elisa Rolle has explained on her blog why she does not give ratings to the books she reviews. It is in response to a question posted in comments regarding a book she sort of liked, but not with great enthusiasm. Link here:
Elisa Rolle - My Reviews and Ramblings
She also tells why when she is forced to give a rating, such as at Amazon, she gives the top by default.
Elisa's blog is worth a visit. Her reviews are entertaining and the eye candy she posts nearly melts the ice caps. Do partake.