Alyson Noel writes YA contemporary, YA paranormal, and now MG paranormal. Lisa McMann write YA paranormal and MG paranormal. Shannon Hale writes YA, MG, and adult fiction. They haven't used pen names, regardless of which genre they're writing.
I think the difference here, though, is those authors are diversifying
within a genre. If they go into a new genre, it might be a genre that'll have a crossover audience appeal or work with an audience that likes a certain voice/style. Meg Cabot, for example, writes futuristics, fantasy, contemps, etc for MG, YA, and adults, but her books go, for the most part, have a similar style - her readers know that no matter the genre MC writes in, they'll have something fun and light to read.
The readers can grow up with the books if they continue to read paranormal and enjoy how the author writes. But, if the author was to switch from FFP to straight historicals, their current reader base may change because the material is going to be different (trading in the werewolves for dukes, the rayguns for swords and all that).
It confuses the fan base, for one thing. I'm not saying the majority of readers are stupid, but if they've read (for example) the Banned and the Banished series over in sci-fi/fan, they're not going to be expecting the Sigma Force novels, over in general fiction.
See, as a writer I want to say, but that's what covers and blurbs are for! If the cover has spaceships and a dark backdrop on it, I'm not going to think wacky contemporary. If the blurb goes on about how Jill knew Jack was the one for her, I'm going to believe romance is going to be a key component of the book. For reasons like that, I rather have an author write whatever they want under one name.
The reader in me, on the other hand, gets bummed out when an author I'm following for his/her action adventure series releases two westerns in a row (a genre I don't read quite as much). I rather just get an email when it has news that'll directly impact me (b/c I'm self absorbed like that) and my reading interests/wallet. 8)
Still though, I rather authors write all their genres under one name. I can decide on my own what I want to read and what I don't want to read. If I pass on an author's current book for xyz reason, that doesn't I'll pass on their next book or some from the backlist if I find it intriguing enough. Covers and blurbs usually play a big part in my book buying decision. And, if the author's someone i really like, and they write something in a genre I usually hesistate to read, well, chances are, I'll try the new genre/book because I think "well, if this author wrote it, maybe it won't be
that horrible..." OTOH, I know people who shop becuase of author name and genre and don't pay attention to blurbs and cover - they just skim for the name and if the name gives them a thumbs up, the book ends up going home with them. Sometimes they like the author's new genre...othertimes, not so much.
Honestly, as long as the releases in the chosen genres are spaced out and I know I won't have to wait a long time for the third book of SuperCoolLikeWhoaActionAdventure to come out, I'm happy.