But it doesn't help you in a trial, and can only hurt you. That's why its not done. It's not hard to teach a dog to heel off and whoa while you mount your horse. But if it goes wrong, it looks really bad.
I have a funny story about that. When my dog was still running weekend gun dog trials, she was running an edge when time was called. About 10 seconds later she locked up. The handler road up, flushed, shot, and heeled and whose her back to the galler, about 150 yards. My wife and aI had dismounted in preparation for putting our pup into the dog wagon. About 20-30 yards everything unravelled. Our pup saw my wife, and could t take it any more. She ran up to both of us happy as a clam. When the handler road up he laughed and said "typical woman, always has to get the last word in"! It was hilarious, and luckily the trial was over. I equate it to a back. You will never win with it, but you can lose with it.