I have torn tendons in my arm from a nikkyo. It will never heal, I will need surgery eventually someday, until then it is something I just simply train on. I find it very shocking that a sensei would act so irresponsible. The tendons and tissues a nikkyo pin is taking advantage of are major tissue groups. If you can't use your arm for a few moments afterwards the nage has in fact done serious damage to the uke. I don't agree people should be "easy" uke, they should in fact be thrown. And yes you should discourage, particularly at the higher levels, uke that thrown themselves. I believe in uke practicing to their nage's level. It is about learning.
Again I find the practice of "pushing the limits" on pins and joint manipulations to be disrespectful to your uke, and their health. If you get a dead arms from a nikkyo, see a doctor. Training isn't play time, and self preservation is a virtue, and respect for your uke's body is required to train in Aikido at all. And it is particularly hard to maintain training schedules, and train to our highest levels injured like this.
To some extent I agree - deliberately harming someone is not on. However in this situation the arm was dead only for a minute or so after - no torn ligaments, no permanent damage - It was a shihan letting an overly compliant uke know that a) he could tell where the limits of uke's flexibility was, and b) don't lead the ukemi. Oh, and uke WAS (and still is) a doctor...