I actually don’t mind transmedia, but my reason may be a bit depressing: it’s because it’s extremely rare for the out-of-game media to make the in-game story better, so I’d just as soon have it tucked away in a book that most people aren’t going to read. I usually find that the story as presented in-game is better than the novel version, when you’d think it would be the opposite, since the novels can get inside characters’ heads.
Example: Thrall making Garrosh his successor as warchief.