My opinion is that more you practice, more perfectly you learn how to anticipate a technique as a Uke to avoid an injury and PAIN. In the other hand Nage is protecting you, and he does it much better when he is advanced student.
So in the end you get an illusion that pain is not needed.
A very good point I think.
In training the nage drives the uke through a "path" in time and space from the initial attack to the final pin or projection. A good nage will be a skillful driver and a good uke will recognize the path that she needs to travel with the understanding that a world of pain awaits her outside of that path. That is the same sensitivity that will allow the uke to reverse the technique she recognize a "gap" in the path