So IO9 has a point/counterpoint about the YA scifi market. This is a huge issue to me, because I am an author who is trying to break in & sell some books. So all other things being equal, if I have the choice of trying to publish something that could sell 3,000 copies and something that could sell 10,000, that's something I'm going to take into account.
From the point of view of a reader, the YA/adult distinction tends to be stupid. Both Phillip Pullman's His Dark Materials trilogy and Connie Willis' DA are considered YA. DA is a fun little 20k-word novella with accessible language about a teen girl who gets sent on a space mission that she didn't even want to go on. HDM is a long, philosophically complex epic with sophisticated language about a girl who is chased across dimensions by an evil priest, ends up killing God & has sex with a boy from another universe. The only two things the books have in common is that the protagonists are kids.
From a writer's point of view, it's necessary to deal with the publishing market as it is. It's equally stupid that Delaney's Dhalgren is scifi and McCarthy's The Road is literary fiction. But even if trying to sell a book as YA increases the sales, could it cut an author off from adult readers? This is important to me, because when I was a young adult, I read scifi that was marketed for adults.
From a purist point of view, you should write what you need to write then figure out how to sell it later. But that's an easy thing to say to someone who hasn't sold a novel yet and is not assured of getting a first book, or a second or third which apparently is even harder than the first.