My only knowledge of the senshusei program comes from "Angry White Pajamas", a book I enjoyed, but is apparently disparaged by Yoshinkan folks. My understanding is that a student can enter the program without having any background in the arts and that the course is (by my definition) brutal. I am neither justifying or defending it. It seems that they take pride in their injuries in that course and that it is considerably different in flavor than regular Yoshinkan classes. After reading the book, I became convinced that I didn't want to train in that program (not that it was actually a possibility) and was very happy where I am. Maybe my limited perception is all wrong, but I would think that crippled Tokyo Riot Police officers wouldn't be all that handy to have around.
I think crippled people were a rarity, but a broken finger or nose now and again isn't uncommon...these are not crippling injuries. While technique is nice, this kind of training forges something a bit more practical than a crisp ikkyo (although it can do that as well). But, as you say, people know what they are getting into when they go there...and the Yoshinkai emphasize safety first, I guess its just a matter of perspective as to what that entails. Sometimes safety is confused with comfort....but as I mentioned in another thread, you have to set the atmosphere of your training/dojo. You can have a dojo of 8 hard core talented students that can't really sustain itself, or you can have a successful dojo where you can teach the art to dozens of students of varying skill levels and varying ideas on what they want out of their art....or a balance somewhere between the two (well I guess the second thing I mentioned is more of a balance than an extreme).