My instructor has the amazing ability to spar at different belt levels. For example, when working with me, he'd tell me he would spar like a yellow belt, and sure enough, he'd spar like the yellow belts in the class. He'd then go up the ranks, and back down again if need be. This way, if someone of one rank didn't have a person of similar rank to work with, he'd spar with them at their level, actually enabling them to get some hits in. Of course, when he got to us higher ranked people, he'd 'kick it up a notch' and pound us around some.
I've seen some people put students through bizarre exercises for workouts that, while they were incredibly painful and difficult to do, they had absolutely no physical benefit. My instructor would always let students know what a particular exercise would develop. If a student was having difficulty, he'd change the tempo or the exercise, or discretely stop the exercise and go into lecture mode, allowing us to rest.
Of course, I always had the problem of not knowing when to quit, especially when grappling. More than once, I've passed out in class because I knew I'd break out of that headlock eventually...zzzzzzz. That, or I'd keep fighting an arm bar or something a bit longer than I should and hurt the joint. I'm a bit more careful with that, nowadays, having premanently tweaked most of my joints. However, I still have trouble knowing when to quit during workouts, pushing myself so much that I nearly pass out. Kind of embarassing.
As far as physical abuse goes, and this isn't just for martial arts, if something is causing undo pain, or just doesn't feel right, STOP. If you have one of those instructors who tells you to suck it up and 'eat the pain', tell him to eat his own @$$. Then find another school with an instructor who knows how to train people. Don't be afraid to speak up if you think something isn't right.
Cthulhu