Sean K. said: "Does the idea of loading up on the backup connote a requirement of strength?"
My koshi days ended (for many years) after a painful groin injury locked up my right leg and my sacro iliac came and stayed out of joint. -- Load somone in a seoi nage, (hook under arm shoulder throw) and your right leg collapses, more injuries, unhappy uke.
It takes something more than zero strength to do a throw - IF you make make a mistake. Guidance becomes bearing uke's full weight across your back if your timing is off. So, one has to be ready to absorb a lot of force if the throw is not perfect. So, I'm suggesting that senior people are playing it safe - and using their superior off balancing skills to make the more stable T type work for them. I don't like the darn thing, but I'm probably just being curmudgeonly.
"Any (koshi) landing you can walk away from...."