In full auto, acceptable. Giving up a little IQ is fair if it means getting the shot. When I'm shooting my 24-70 on the 5D2, it's nice to have the extra reach plus I can bracket exposures. When I'm shooting my 7D with my 70-200 or 200 f/2, it's nice to have the shorter focal length or to go up to 1200mm equivalent. It will work more as a supplementary camera than a backup.