I learn a lot simply by taking ukemi from my sempai, too, so I know how it's supposed to feel.
I have to agree with this strongly. In training with the higher ranks you can emphasize a more realistic attack. I find that when I provide a firm grip from a static position, I can really get a feel for how I'm being moved by most Yudansha. As far as I can tell, the best way to learn is to take ukemi from someone who knows, and to try to feel the technique. I have been trying to get into the habit off asking instructors to "please, let me feel it" whenever I can. Usually they're cool about it.
On the other hand, its usually the ‘noob' that can provide the least lenient of all attacks we can hope to receive; as we progress from mudansha to yudansha, I suspect we gain a certain expectation for uniform attack that simply won't exist on the street. Perhaps what we can learn the most from the new guy is that most people out on the street are going to throw a punch exactly the same way he does on his very first day.
Its probably also worth mentioning that when you have to explain something to someone you usually have to be able to make it work, so it'll keep you honest.