One-star reviews are written by assholes. Five stars some from friends or people are often too excited.
When I look at reviews, I discount the fives and ones. But if I see a book heavy with three stars, that's a book I might not care about. One-star people care. Five-star people care.
A three-star review says "Meh. It wasn't great, but it wasn't bad. Whatever." Who wants to read that book?
And then there's this kind of review, one that says the book was great and the author did great work, but it wasn't exactly what the reader wanted.
What the heck are you supposed to do with that?