I have no real life experiences to fall upon but at my dojo our instructor makes a point of having us change partners with every new technique. It is invaluable in my eyes and I quite agree with all the original poster has to say here. I feel more confident in my techniques after a class where I've had to take on an uke with strength and bulk, or slight of build and very short or someone short and strong. The techniques must change slightly with each body type and had I not the chance to practice with different people I would not know this small, but important fact.
"Professionals are predictable. It's the amateurs you have to watch out for."
That is an interesting comment. I'll take that on board.