The Borders Blog for Science Fiction and Fantasy had a post up around wanting to split up Science Fiction and Fantasy stories. It's an interesting discussion, but it misses a lot on the mark about the types of stories that are around in the genre, while also completely missing the entire point about genres in the first place (which makes this really funny for a major bookstore blog), which is to say that genres are purely a marketing tool that are designed to put a certain product into a clearly defined audience: the speculative fiction fan.
Books in a bookstore are marketed based on the elements of the story, and are essentially grouped together based on what the characters experience, rather than the story type. Thus, bookstores are all predicable marketed as Mystery, Speculative Fiction (which is a horrible term that pretty much encompasses... everything in fiction, but really stands for Science Fiction, Fantasy and Horror novels), Romance, Literature (upper tier fiction / classics), sometimes Christian Fiction, sometimes poetry (although that's sometimes lumped into classics), sometimes Westerns, and then all the various nonfiction categories, which are arrayed by topic. Beyond the marketing element, genres are essentially meaningless constructions that should have no impact on the reading of the book.
There is not a whole lot that separates the two genres from one another, which makes this argument somewhat confusing. Splitting Science Fiction and Fantasy apart simply because there's a perceived, and false, notion that science fiction authors hate fantasy, is a ridiculous notion, because it's overly simplistic and not something that I think has any bearing on the actual books in the respective genres. From every author that I've ever read, spoken to, or listened to, there is an understanding that fiction is primarily about storytelling and the characters within said stories. Very few authors, I think, will set out specifically to write a book because it will fall within the science fiction genre. They might have a good story that falls specifically within the science fiction or fantasy genre, however, and the distinction is that the stories and the genres themselves aren't uniform blocks of good and bad. It's a pretty shortsighted statement to say that you hate a genre as a whole, simply because it has magic or other fantastic elements in it, or for any other reason. Looking at other genres, it's highly unlikely that you'll find a unified block of writers that like or dislike any other genre within the general fiction heading, and undoubtedly, you will find various groups of authors and fans that dislike certain subgenres within the larger genres.
This past weekend, I attended ReaderCon, and attended a panel around interstitial fiction, which primarily defines the stories that fall within the genres. It was an interesting talk, and largely boiled down to: there are simply some stories that are indefinable, because the stories have elements that move between both genres. There are major, general trends within science fiction and fantasy, especially concerning their outlook on the world, but these are not universal, and ultimately, the definition simply defines where the book is placed in a bookstore. One panel member at the con, Peter Dube, noted: “If there is no pleasure in the text, I won’t read it.”
At the end of the day, it is those two things that define the genre: the buyer, and the bookseller. In general science fiction, fantasy, weird fiction, horror and gothic fiction and all of the others generally appeals to a similar audience, and thus, everything is marketed together, which helps both the buyer and the bookseller get what they want: a good read, and a sale.