Personally, I do prefer everyone to teach if only to find the gaps in their own knowledge. However, teaching during a class and actually being the main teacher is very different and if someone isn't comfortable (or capable) of taking the class, I'd prefer they didn't do it.
I actually like teaching and find it a buzz. I also find my own aikido suffers if I don't teach from time-to-time. Somehow teaching negates some of the sterility of doing basic ikkyo for the umpteenth time and breathes fresh enthusiam into me. Teaching aikido isn't just a one way street and I'd definitely recommend it to anyone staying with aikido.