I have seen a horse with those tooth rings and they are horrid. The man held the horses mouth open so I could see, and he bumped one of the metal rings, and the poor horse jumped from pain. I would never do that to my horse.
My horse is a cribbing addict. He is the worst cribber you will ever meet. His teeth are almost gone also, even tho we have a collar on him and he can't suck in he will stand there for hours trying to crib. He is so bad that as soon as you take the collar off he will try to push you away and get to anything he can put his teeth on.