I wonder if, to some extent, we choose the books we do based on the covers anyway. I know I hate reading books with a picture of actors once its been turned into a movie -- I'm sure a lot of us do. Of course it takes it to an extreme to say "I'll only read books with covers that appeal to me," but we do it anyway. It's become cultural shorthand to see a book with a pink cover, an image of an engagement ring or a baby stroller, and know that it's "chick-lit." We know that gothic letting and an illusion to teeth and/or blood is probably going to grace the cover of a vampire book. Judging books by their covers saves us time of having to research each and every book we pick up to see if it's something that would appeal to us. Conventions save us time and mental energy.
Of course, there can be both good and bad covers that fit genre conventions, and I know that's more what we're talking about here, but I wanted to issue a challenge: please find a well designed romance novel. Preferably with Fabio.