OK, let me posit a slightly different perspective...
Uke means "to receive". Assuming that the law of reciprocity applies (work with me here for the moment)... in order to "receive", one has to first "give". So uke attempts to put force into nage... but nage isn't merely a conduit to the ground... nage is going to not only reflect the force given, but also add to and amplify it (i.e. aiki) on the return (what Mike is saying in relation to baseline skills)... sooooo..... if uke cannot handle (absorb/redirect) the amplified force on the return what happens? Possibly a roll out or a break fall (the simplistic meaning of ukemi)? (I'm talking of course, within a training
And if uke can handle the returned force (the literal meaning of ukemi), it becomes kaeshi (within the context of aikido) - and perhaps, more along the lines of what Dan is saying?
So, to me, I don't see why ukemi (in the literal sense) is not (or cannot be) related to kokyu development, since uke is also learning how to express and receive force, within that context.
It is a bit of a paradigm shift, because the delineation of nage/uke is blurred, insofar as uke is able to continue to receive and return the force to nage, and vice versa. I think it is important to clarify that this is "play" and not about winning or losing, but a shared learning experience.
FWIW, I take ukemi for all my "students"... and depending on where they're at in terms of understanding and ability, I might choose to "fall over" or "roll out" (whilst still maintaining a viable structure), OR, I might still take "ukemi" and "push" them just a little bit more. Even if I do take a dive, I might even take it to the ground (but only if they're really cute).