From another angle, this is an art vs. commerce problem. As a writer, I don’t like artificial boundaries. As a reader, I don’t like artificial boundaries. As a librarian or book-buyer, however, I consider them essential. Without them, it’s really hard to find the things you like, because you’re sorting through piles and piles of books with little or no guidance. In any case, get on over to Robert Jackson Bennett’s place and check out his work.