I find that the more aikido I learn, the more grateful I am to my teachers and the more I want to be able to teach others in return. Yes, I do believe that training with beginners is beneficial and that in teaching others, I am also teaching myself. But the circularity is wider that the nage-uke or teacher-student role.
By passing on what my Sensei has taught me, I am showing respect to him & his teachings... this flows back to O'Sensei. By teaching others what I have been taught, I extend the flow into the future.
Hmmm. I'm not usually of the metaphysical bent - but there seems to be some sort of ki analogy here
On a more practical level: A dojo would not exist for long without a steady influx of beginners. We all know how many drop out along the way, or take time off to start a family etc etc. If we don't train with beginners and help them become more advanced, we might be left to train with our shadows.