I think it's the instructor's duty to select Uke that will competently assist in teaching the required principle that is being dealt with during the session. This includes their level of ukemi, relaxation levels and ability to adapt to sudden unknown movement changes without losing balance easily etc. If I am teaching a class and my Uke continuously resists my technique during instruction for no other reason than a test of strength/wills then this to me is a serious discipline problem, regardless of whether or not I throw him with another waza. I have Uke who give hard honest attacks during the teaching phase and imo this is good because if I can't do the technique on an honest predetermined attack then I probably don't understand the principle well enough to teach it anyway. This is not the same as resisting just for the hell of it imo.
During randori of course and other types of training I invite folks to resist at will, but during instruction one should understand that what is being shown is for the benefit of the entire class and by not cooperating with the lesson demo they are not only cheating their partners from learning something they are also cheating themselves from developing good ukemi skills. At last night's session we worked precisely on this by showing how an improved level of ukemi is necessary for effective application of kaeshiwaza in a resistance-based environment. I think it drove home the importance of Uke's role (especially relaxation and timing) to those who liked to resist everything.
Just my 5 cents.