I don't know if you have ever used one of these for rapelling. I have and it's scary because to rap down.... you pull the lever back and you freefall intill you release the lever and you stop instantly. BAM right on the anchor!!!!!!!!!!
That's not necessarily true - you can get a smooth rope feed and stop, although it is tricky and takes practice. However, relying on a knot jammed against a rappel ring ( without that backup carabiner ) was the cause of the accident - not the use of a Cinch. And was an admitted mistake ( see the Supertopo thread ).
Your comment on carabiner brake rappels is incorrect too - they are perfectly safe when done correctly - I used them for many years, and wouldn't hesitate to use one now. Everyone should know how to do one in case they drop thier ATC !!