Bill, the thing is, I find I learn nearly as much assisting beginners with their training as I do practicing with fellow established students.
This is an oft-repeated aikido truism. I'm not challenging it, exactly, but I'd really like it if people would elaborate on it and say something about exactly what they learned from assisting a beginner -- and then, what they learned from assisting the next
beginner making the exact same mistakes. I think the truth is that there are occasional insights to be had from teaching beginners, but if we're being honest and realistic, there are also times when the experience is just a lesson you've had before. That's life, that's training, and I have no beef with it -- but I also don't think we need to pretend that the experience of training with a beginner is always a golden opportunity just waiting to shower epiphanies upon us.
As for the beginners themselves, I think we can distinguish between "acting as uke" and "learning to take ukemi". A brand-new beginner needs to learn to take ukemi, and there's no other way than to do it - but for a while, they're not functionally "acting as uke" (as nage desperately tries to keep "uke" from hurting him/herself or patiently guides "uke" away from a situation that would lead to one's clock getting well cleaned in a hostile situation).