One rein stops are useful, but they need to be done right. I really only use a one rein stop to keep a horse from bolting or if he is not listening to me, I will one rein stop him and then start flexing and disengaging the hindquarters.
In my experience, I think that flexing your horse laterally from a standstill then asking for him to move his body (front half independantly or back half independantly) has created a more responsive horse for me.
The one thing I don't like about one rein stops is a lot of times I have seen them to let the horse drop his shoulder and put his weight on the forehand. In barrel racing, some girls will use one rein stops to teach their horse to "bend" and well...that just causes more problems later on.
Good topic though. And I want that poneh.