Ok I have actually see some use this feature with a horse. And the man that did it is a well known trainer. He had my gelding ready for me to pick up and 1 of the requirements was that we would go on a trail ride. They had 300 acres of woods along the Mississippi. Someone had gone into his wood and had put no hunting sign up on his property. He wasn't sure how they got them up that high but as part of our route and not on a trail every so often he would take the horse he was training and sidle him up to a tree, stand up in the saddle and pull the signs down.
Now as far as in the sale barn if I see a person do the stand up on his back, the main thing it would tell me is that they have spent time with this animal and this horse has been handled. Would I do it probably not, but you got to admit that it isn't something you can do on just any horse. And if you are buying a horse for a kid odds are they will try it. I know I did.