The fact that everyone has a different way of rating is argument enough not to give a star rating or to pay attention to them. If you gave 5 stars each to Beloved or TKaM and a solid piece of genre fiction, for example, how is that helpful, since so much of good genre is still ephemeral and won't stand the test of time? I also find genres to be self selecting. So someone into Manga will give 5 stars to many things Manga, which isn't really helpful to someone who generally reads another genre and wants to try the best of Manga.