I think that on a practical level the problem with defining science fiction and fantasy as opposed to other genres is that the genre of sff doesn't fit neatly into a marketing category and thus on a day to day basis when trying to find a particular work or even browse this genre in the bookstore I (and probably other readers) run into problems that I don't find when browsing Mysteries or Romance or other categories or genres. I might find Oryx and Crake and The Handmaid's Tale put into literary fiction. Vampire stories are sometimes put into Horror. And what about magic realism? It's put in literary fiction usually, but from a genre defining perspective I don't see why it ought to be. I have found many books I consider science fiction and fantasy in other marketing categories. What would be really useful for me is for the definition of the sff genre and the definition of the marketing category of sff to coincide.