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.
Mary, the reason I learn a lot from working with beginners is that I can focus on doing things perfectly. It is a chance to pick apart my technique, but as uke and tori, and remove every bit of unnecessary movement, speed and energy and focus on the most basic (and thus most important) aspects of my training. I have to refine my understanding further so that I can share it with the beginner at their level. For me, it is never an experience I've had before, because I'm coming to it at a new place in my training, so whatever I have learned since the last time I was there will be tested, refined and polished. If I am acting as tori, I use the least powerful grip and the most minimal connection I can while still being able to perform the technique. If acting as uke I can practice and experiment with subtle changes in body structure and placement that I frequently don't have time to practice with more senior students who want to go at a faster pace (nothing wrong with that. I need practice at faster speeds too). It is also a chance for me to work on my awareness of tori in ways that feedback to what I do with senior students. When I work with beginners I can force myself to train at a minimum of energy to find out where that is, and how I might be able to apply it to more senior students.
Does any of this make sense?