I never get where this opinion comes from. Since his return to the X-books, Claremont's always been good sport about using what other writers create -- to a surprising extent, since writers returning to a book frequently sweep aside what came in-between. But instead of doing that, he gave significant roles to characters created after he left like Cecilia Reyes and Bishop, and during his run on X-Treme X-Men, he frequently not only referenced what was going on in the other X-Men books and built plot bits off of them. Here's a game: Compare how often events from X-Treme X-Men were brought up in the other books to how often events from the other books were brought up in X-Treme.
I wasn't a fan of a lot of those stories, but not trying to play with other people's ideas wasn't one of their flaws.