Since I write "Between the Cracks" I would say don't split it up. I dislike genre labels anyway and many of my stories would fall under several labels or none. Besides if you start splitting where do you stop? Where does dark fantasy end and horror begin? What about Magic Realism? Time Travel? SplatterPunk?
I'd say a hindrance, because sometimes it's not so easy to pigeonhole a story. I know the things I write are not neccessarily easy to fit into a genre. On the other hand if the group gets massive and really busy, then it might prove neccessary, so long as you include a "No specified genre" category.
Since all these elements blend into each other as parts of the whole, I would say don't split them up, unless someone wants to segregate comments by subject, like dragons for example. Otherwise, anyone wanting to find new material to work with would feel excluded. We don't need to put blinders on. "Frankenstein" was always billed as horror but was really science fiction. "The Andromeda Strain" was science fiction, but had some fantasy in it. "Flowers for Algernon" was billed as science fiction, too, but also contained some horror. So you really cannot distinguish some fiction as being in one specific genre or another.