I think there are several facets to ukemi practice - depending on what the situation is and where people are at in their practice. The real issue I believe is that the view that nage is doing "something" TO uke (i.e. throwing uke) is incorrect. I believe what makes aikido AI-KI-DO is that uke and nage are working on themselves, whilst at the same time working WITH each other.
Mark C. mentioned the center-to-center connection (more accurately, whole-body-to-whole-body connection), which I believe is the crux of it, or at least the most important aspect of aikido practice. If uke bails before the "moment", nage "knows", because the connection is lost. If uke "resists" or switches, nage also "knows" that their timing is off. It all depends on where people are at in their individual practice that allows them to either "know" or "don't know".
The issue is how to convey that to the student at the appropriate moment and allow them to become self-aware of what they need to do to change their level of practice. That, I believe, is the role of the sempai, instructor or teacher.
Which brings me back to Mark C's dichotomy of training levels (and me not wanting to steal his thread thunder).... it is the role of the teacher to assess where the students are at in terms of skill level and to constantly push the boundaries of their level of practice such that they aren't simply taking a fall to merely "protect" themselves or to make nage "look good".
PS: I also meant to add, that nage and uke are both practising EXACTLY the same thing... ukemi...