Exactly what I was going to comment – you need to make mistakes in order to LEARN.
If the rider only can ride the horse when the horse is doing it right … how is the rider going to know what to do when the horse makes a mistake? Or the rider makes a mistake?
I don’t like the theory of “don’t even make a mistake” because that’s just not realistic.
I do see your point on how the trainer needs to have their clients perform well … but at the same time, the trainer does not OWN the client. I think there is a difference between the trainer showing their own show horse that they own, versus a client showing the clients horse under the trainer’s guidance.
It doesn’t sound like the OP wants to go off and ride his/her horse for weeks on end without said trainer – just wants to be able to ride here and there when the trainer is gone. I don’t think there is anything wrong with that and they aren’t going to do serious “damage” by doing that on those small occasions.
Obviously, the client needs to be able to show the horse in the ring without the instructor there to guide them … so why can’t the client ride their own horses from time to time? Consider it a practice run without the instructor so they can learn their strengths and weaknesses when their instructor is not there.
Good points on either side. Ultimately, I think the OP needs to have a conversation with the trainer!