In many aiki-dojos, new students are supported by the most advanced students in class which means Yudansha in most cases.
I am surprised to read Bruce's post. In my dojo in Kyoto and in my dojo here in Israel members assist me teaching with great pleasure. I never heard of an Aikidoka who quit because he didn't want to teach.
It is very sad if after a long relationship with the sensei and fellow students, one can just leave without talking about it first.
I see in Israel many people who got their shodan and quit their dojo just to open their own Aikido class. Whenever I talk to such people I encourage them to keep on learning and find a sensei whom they can learn from.
I saw a program on National Geography about 8 dan test in Kendo in Kyoto. They showed a 84 years old man who is 7 dan who goes to learn from an older man who is 8th or 9th dan.
How inspiring. How I hope to see such approach in Aikido too.