Disclaimer:
If you WANT to go win at reining, which can be really fun, please disregard the following.
A horse cannot make an athletic move after a cow, if he learns to use the inside hind foot to pivot. He’s going to be a beat behind, always. In order to be able to leap out after a cow from any point in his spin, he has to make that spin like a dressage pirouette, which is to say using the outside hind as his pivot point.
Of course, a horse in a reining class will NEVER be asked to leap out of a spin to gallop after a cow.
Most ‘Working Cowhorse’ classes (Snaffle Bit Futurity) will forgive a horse that doesn’t ‘spin like a reiner’. These horses won’t spin that way, because it pretty much trips them. A ‘reining spin’ is a ‘pirouette’ on the wrong lead.
If you do reining spins a lot, and fast, you can be very hard on a horse’s stifles and hocks. Not for nothing are reining horses’ joints very commonly injected. I was really sad to hear a friend’s story about her 3 year old, who was lame and therefore injected (stifles and hocks) a month before the Futurity. Her trainer said it was almost expected, it was unusual not to have to inject one. Sigh.
If you are doing a spin as a stylized, fun trick for a reining class, fine. A long sliding stop isn’t very useful on the cows, either, but it’s fun to do and fun to watch.
If you don’t drill the enjoyment out of the horse, I don’t have a problem with it, but if your aim is to be good at working cattle, I wouldn’t train a horse to pivot on his inside hind for a ‘spin’ on purpose.