Forgive me if I'm wrong, but you seem to be coming in with the assumption that most tie-in fiction is by definition inferior to "literature" or even mainstream science fiction. I know that sci-fi isn't the most respected literary genre, but to dismiss huge amounts of published work like that is quite unfair.
That was not my intention. I am a seasoned reader of both sci-fi and contemporary literature. As I said, I have not read very much tie-in at all (though the little I have read, aside from the Moorcock novel I cited, is risible). I posted this query to learn.