I always felt that this advice, when it appeared years ago, was written by someone who had never driven an automatic off road and was drunk at the time and thought it was a good idea.
There are 3 main reasons not to do this:
1) You are in great danger of destroying the torque convertor. 2) If the slope has too little traction you have no way of using the accelerator to get you out of trouble. You have no way of, in effect, releasing the brakes so if the tyres start to slide you can't drive out of it. This is FAR more dangerous than a lack of engine braking. If you fancy going down a wet grass slope with your wheels turning in reverse, please let me know so I can have a video camera ready :-) 3) In a trials competition you're not allowed to stop.cheers
Dave W.