Bill, the thing is, I find I learn nearly as much assisting beginners with their training as I do practicing with fellow established students. As I noted, the beginners will progress far quicker with good ukes than they will with other beginners. If you want good partners, you have to actively train them. And I always remember that Omori Sensei (8th dan iai/7th dan kendo) always took the beginners to teach. He focused on bringing the lowest in the dojo along the fastest. He would train everyone, but he took special care to focus on the new students. The result was they learned quickly and didn't stay new students for long.
The care and attention might also contribute to a higher retention rate.
+ 1 on all of this.
I learn a lot from how my technique is actually going by slowly practicing it on someone who has not had their body reflex 'conditioned' to it.. so they do not move 'as expected' unless I do everything correctly. It is a wonderful opportunity to practice and refine the lines/angles of your waza.
I am in a similar position as Bill with feeling like I need to focus on Shodan grading etc... however it is activity encouraged and practiced to not let new people train together and more so to stagger the relative experience as much as possible. So higher kyu's pair up with with new people and when they are taken care of, then lower kyu's will pair up with any remaining higher kyu's. The result is generally well balanced and the 'middle kyu's' will sometimes end up together. In the end, I get a lot of valuable information regardless who I train with and it is all useful heading towards Shodan.
It is a very enjoyable way to practice.