That's true. That's why some writers expressively forbid others to write books set in their universes (I'm thinking of Zelazny's Amber). I think that Brian just wanted to keep his father's legacy alive, though, and I still haven't read the two books that conclude the saga (hunters and sandworms, if I recall correctly) which should be based on his father's notes, and therefore should contain interesting elements. If you're planning on reading the 6 original books you might want to try and see how it all ends :)