I'm going to call false dichotomy on this, with a generous dose of armchair quarterbacking. You've reduced the situation to two clear-cut choices: using an uke that "understood the point" (assuming one was available -- this was, after all, the very point that you are trying to teach, and you would hardly have been trying to teach it if everyone already understood it), and using "someone who is more likely to hurt themselves" (assuming that you have prescient knowledge that this person would act in a manner directly counter to instruction and against the feedback that their body is giving them). Reducing a situation to two choices is attractive for the purpose of constructing an argument, but it's also often simplistic. In real life, the choices are seldom so clear, nor are they so neatly divided into exactly two choices. It seems to me that in this situation, having only one choice, or having several choices, are at least as likely as having precisely two.