I agree that the Ushiro attack is a pretty silly attack. I believe that I read somewhere, probably in a John Stevens book that Osensei believed that it was unsound to let someone get behind you but that Ushiro taught that one can still recover if they do. Also, to just stand there like a tree seems tactically unsound as well. I think of Randori and Ushiro similarly in that way. In that context, the attacker/s have to grab you where you want them to or at least where they are able to and not just Teeing off on where they want to and how they want to.
O-Sensei stated that (and this applies to allowing the uke to actually be behind you and initiate an attack) ushiro waza was about training your intuition. By that he meant training your other senses to perceive an attack rather than simply using just the eyes.
The Ura practice of the uke starting in the front and attempting to slip to the rear, resulting on our beautiful spiral waza, is a connection exercise and is not representative of applied technique. It's more a test of ones ability to connect and stay connected through a complex series of movements without leaving an opening for uke to counter or strike you.