i believed this is where koryu model worked better. if my understanding is correct, in koryu, uke usually was the teacher or senior person, which meant uke was better. and as the senior person, uke knew when, where and how to resist or comply to the pin to teach nage the right way and the wrong way; yet as the same time can protect himself/herself from overly excited nage. however, aikido model, at the moment, isn't the case. i kinda like the systema approach; they just hit you wherever you try to resist. that's another thing. aikido folks don't like to hit folks when they are down. rolling around with the systema folks and they will dispel that notion in a hurry. quite enlightening experience.
IMHO, That kinda depends on the lineage you are in and how you practise. I agree that most often you will find highly stylized techniques and a set of rules on what you can or cannot do. (Lot of this has been discussed in the power
Dan said it would be wise for teacher to take ukemi all the time. This works two fold, on one side you will feel what your student is trying to do, on the other hand you learn much more about your own control as uke (like you also mentioned). I have started doing just that to find out what happens... it has been interesting so far!
Before this derails too far I will stop here...sorry