IMHO, ukemi really teaches the giving of one's self for the benefit of others. As a good uke, as a good training partner, we interrupt our own training and lend our body and energy. We attack/approach/enter knowing we will fail/fall, but do it to benefit others.
I also think that being on the recieving end of a technique/waza really gives a different and valuable perspective to the move.
And lets never forget the humility of being thrown by ohers much smaller and using very little effort. Sometimes, I just lay there and laugh.