Here's my 2 cents' worth:
I've never been *required* to teach, so I can't speak to that. I would say that is kind of a sketchy practice at best, but hey....different people have different methods.
All of the Black Belts that I have known as teachers *wanted* to be a teacher. My former head instructor/school owner worked out deals with everyone who was a teacher: as teachers, we got our tuition free, as well as got paid by the hour (not a whole LOT, but hey, classes were free!). I think that work ought to be compensated, definitely -- I think anything else is a little shy of legal, but that may just be me...
We had a "student teacher" program, more or less, where students who were high enough level (brown and up) could train to be teachers by being assistant teachers: helping lower belts one on one, holding bags, etc. while observing/shadowing the teachers. That way, they got training on how to not only do the techniques better, but how to teach them better.
As for the reasoning behind why one would want to teach, aside from helping to make the art really *live* for students, it also helps the teacher deepen their own knowledge of the art (at least, if they are paying attention). That's another one of those zen things -- "A good teacher can learn from his own teaching". I have had many "AHA!" moments when teaching....where a move/technique I've been doing forever suddenly takes on a whole new light....
Basically, I don't think it's right to make teaching a *requirement*. In some cases, that may do more harm than good....just because someone is good at something doesn't necessarily mean that they are qualified to teach it right.
Your mileage may vary.
Peace--
--Tonbo