The rock climbers I associate with -- Texas Mountaineers -- teach the use of an auto bloc to back up the rappel. Some of these guys are very experienced, and I think they would say that is the standard in "their" climbing experience. I think canyoneers disparage use of the backup too much, thinking they don't need it because of their expert technique. I think if Hank were to chime in, he would say it definitely has a place in the tool box. Just don't take it out every time.