Well, in all honesty, bucking out horses was the old cowboy way of breaking a horse, so has some credibility. And too...some horses are just going to be buckers anyway, they might as well be put to some use...I guess. I've always believed that bucking one out doesn't make the best horse for riding later, but I don't necessarily see it as cruelty either. Close, but not entirely. It's screws them up more mentally then really causeing them pain. I've heard of several bucking horses that did become riding ones after they were bucked out. Rodeo Cowboys are suppose to bail at the 8 second timer even if they can stay on, so as not to buck the horse out. But after a while I would think some would figure out that not bucking is better, that is when they get retired from the bucking string.
Becky
(I know you know all this) It would only resemble real life skill if they did try to buck them out till they stopped, but it doesn't make good enough sport, so they try to make them worse. That's why it's a perversion of the cowboy life. What they do doesn't resemble the way you would break a horse. On the ranch there'd be no bucking strap, there'd be no attempt to make the horse buck more and harder, they'd encourage the horse to run and move forward till he got tuckered rather than buck.
The horses retire to slaughter when they stop fighting. I can't imagine them making good riding horses after that start either.