I mean there's any number of reasons.
You could want to see their dynamic with other characters.
You might want them to flesh out an aspect of their character that gets no play in the X-books.
You might want to see them get used, which they don't in the X-books.
You might want to see their fresh perspective in an area that's not X-book related.
Them being Mutants doesn't preclude their ability to do other things, or be elsewhere. Hell there are a handful of mutants that've never even touched an X-book.
In this thread alone you've got people pointing to the excellent treatment characters like Roberto, Rogue and Beast have gotten outside the X-books. I don't see how in any way it's a negative to want the possibility of a favorite of yours being treated better, or being treated at all elsewhere.