Jason,
You know, your replies to things I've written are usually closer to what I had in mind in the first place, except from me it becomes more like a regurgitation of thoughts. That's why I've tried to write about this lately. See if I can be coherent.
If there's no longer any challenge in something, why keep training? You're a master.
What I'm trying to get out of my gray cells, is that the new challenges are usually brought upon by ourselves. Challenges given by uke, to me, are pretty much the same from the day I began. The experienced uke who is constantly refining her skills will still be offering the same challenges ... they just become more ... well, refined. :-/
I never said they ceased to exist.
I wrote:
While training, uke and nage need to take care of each other, yet worry first about what they themselves need to work on, not what they think the other person needs to work on.
then Jason wrote:
At yudansha level I would agree with you to some degree. Below that I think it's selfish and ultimately counterproductive.
Actually, I believe it's beneficial for all to train this way. And I don't believe the approach I referred to is necessarily selfish. Perhaps I should have written, each person should take care of each other, first. Well, at least I wrote it first.

After that, if what I need to work on as uke is to focus the intent of my attack, then nage benefits (although its not always about nage). I wasn't implying to train in a vacuum where only one person exists. I guess it's a ... from brain - to word - to interpretation kind of thing.
For uke, particularly a senior, to practice with nage and give no thought or attention to what they're doing or how they're moving is wasting an opportunity.
And
that would be bad ukemi. I wouldn't and didn't suggest otherwise. What uke should always be working on
are those things. That's what I think ukemi is all about.
There were plenty of people who went through the motions and "fell down when they were supposed to".
Depending on how you interpret that, that could be correct or incorrect. Now we're getting into something here that I know I can't explain with words alone. But I know what you mean.
Otherwise, you're just contributing to the development and prolonging of bad habits and dead ends.
And this is something else I would agree with. Just from the other end of the spectrum, I think.
I think where we're ending up here, is perhaps the same place. What I'm trying to get at here is if uke does basically what they are meant to do - attack and fit, and protect themselves ... THAT can be one hell of a "teaching" challenge for uke and a great learning experience for nage. Anything else that gets thrown in
occasionally is also beneficial and natural.