Or maybe they just don't give the symbiont to anyone else. I don't remember now.
Erza being implanted with the Dax symbote was a emergency situation, Dax would have died prior to arriving on Trill if not implanted in someone, anyone.
Ezra (iirc) wasn't in the symbote program and didn't receive the training.
Oh, ok, makes more sense now. I must have not been paying attention there. ^_^