Jokes aside, every artist is entitled to have their comfort zones and knowing what their abilities are; heck, even Brian May didn't want to play the flamenco solo in Innuendo and they got Steve Howe from Yes to do it.
Having said that, what kind of reply from a professional artist is "it's not the thing I usually do?"... imagine John Petrucci being asked to be a guest on an album and saying "nah, I'm more comfortable doing prog metal, I don't want to play a rock song".