Re: Overly-aggressive instructor, anyone?
There are times when it's appropriate for an instructor to ask a tired student to continue, but it has to come from a relationship of earned trust. And I would expect that if the student flatly said "No, this is unsafe for me" his/her judgement would be respected. (The last time I had to say that, I was actually praised for good judgement afterwards. It may have been a test of sorts.)
Continuing after the student has tapped, though, is an invitation to injury. If this is truly the "only game in town" I would see the viable choices as creating a new game or quitting. Allowing abuse to continue is not good for you, for the instructor (who doesn't receive the reality check he may need in order to change) or for the art in general. Even if you cultivate the ability to protect yourself, what about newer students? And what kinds of bad habits will you pick up?
At one point we had an instructor with an overly painful approach to nikyo, and to this day my teachers are wrestling with students who tense their arms and lock their wrists inappropriately when receiving nikyo because they still expect excessive pain and want to protect themselves. Those of us who missed out on this experience learn faster because we have fewer hangups to overcome.