Good point Jun. I do pretty much the same as you - that is, practicing with whomever is sitting next to me. I know of some students in our dojo who are constantly trying to position themselves during class after each technique so that they can work with certain sempai next. To me it's simply not worth the effort, since I don't want to be bothered looking for my next partner while I'm practicing with my current one. I usually get something from every partner I practice with at this point. Maybe I'm just spoiled from practicing in a pretty well-established dojo...I don't know.
As far as beginners being able to recognize good technique, I remember as a beginner (and now) being able to recognize students whose technique was better than mine (which wasn't and still isn't hard). Whether or not they were yudansha wasn't an issue to me, because we could count on the instructor to help us out if either one or both of us was having trouble with a technique.
Might make for an interesting poll, though, don't you think (ie, how do you pick/find/select your partners for practice)?