I'm not a fan of them either. There was a horse at my camp a few years ago that someone must have thought would be funny to teach to spur stop (he was probably a TB/Clyde, but definitely some kind of draft cross) and do it badly. Well, this horse had no concept of leg pressure meaning anything but STOP. Any leg touch meant to him go slower or stop. It was really ridiculous. I spent the summer trying to retrain him out of that and I really wasn't able to get far since his natural inclination to move at leg pressure had been thoroughly messed up (I also wasn't very proficient with horses yet, semi proficient but not enough to be doing any sort of retraining).
And, on top of that, he was a spooky horse and when I'm riding a spooky horse, I like to keep a little more leg on to kind of "reassure" them that I'm there and that they'll be ok. Well, with this horse I had absolutely no way of telling him that he was ok from the saddle. And since he was at least 17hh...Let's just say that I tried to ride him as little as possible. However, he was the one horse I've ever ridden that made me look short. Haha! He was also a really comfortable ride, through all that, just VERY spooky.
However, sad day story: 2 years ago he fell out of a trailer and broke his leg and in xraying it, the doctor discovered that he hadn't gotten enough calcium as a youngster (or something) and that his bones were super brittle. So even though it wasn't a bad break, he was immediately put down. :(
Anyway, I'm sure when it's trained correctly it can have it's uses but I really see no need for it. Why not just use "the seat stop"?