Being a good Uke is important for the growth of the Tori and yourself. I have always been under the impression that it is my job as Uke to sell the technique and help assist Tori with learning. It helps everyone when resistance is applied. Obviously most Tori's will not be as proficient as the Sensei but, we are all learning.
I agree we help each other learn but disagree that resistance is part of the learning process.
To use an instructor's analogy: do we help beginning drivers if the instructor in the passenger seat applies the brake every time the learner in the driver's seat applies the accelerator?
I believe my job as uke is to give honest feedback with my body by going exactly where tori is putting me. With a beginner I will stay a little ahead of tori in order to help guide them IN the right direction and magnify this feedback so they can see and feel where WE are going.
For non-beginners, if their intent-driven small movements are beginning to have an affect in shifting my weight, I let my body move with that (rather than readjusting or regaining my balance) so they see and feel they are on the right track and don't feel they have to add muscle to "make" me move. I add verbal feedback too if it seems needed - "you have my balance now" or "you disconnected there."