On a recent visit to the cinema I watched the trailer for Whiplash, a movie about a young jazz drummer and his tyrannical, bullying teacher. The film is asking the question how much 'pushing' or even bullying is justifiable to get an outstanding performance out of someone.
I once participated in a theatre workshop with an internationally renowned avant-garde theatre director. There was a sense of the teacher having absolute authority and of students having to put up with some degree of unkindness, such as being mocked or being ignored. It bugged me a lot; I did not get why it was necessary. I certainly didn't learn more because of it.
In my heydays of physical theatre training I sometimes had my teachers yelling at me to push me just that extra bit. I didn't mind, because I really benefited from it and it was done by someone who I knew was generous and supportive towards me.
So I suppose the question whether the end justifies the means all depends on the context? Or does it? When is being pushed in training or rehearsal a good thing? When isn't it?