Yep John, I see where you are coming from but you have to realize that most of us "non engineer" types would not think that way and would most likely react the same way I did.
My thought was that, without the bars, the coupler would just pivot on the ball as the trailer backed up the hill. Never gave a thought to the "push" created by the weight of the trailer and I still have a somewhat difficult time picturing that in my mind unless the trailer was almost standing on its nose.
Not arguing with you at all but just having a hard time understanding the difference in forces between that and the pulling down of the WD bars on the A frame of the trailer. In my "non engineer" mind, I see the force on the A frame (truck lift?)as much higher and more dangerous to equipment than the force applied to the trucks receiver from backing up a hill or a driveway on a grade without using the WD.
Glad to see you joining in on this discussion! :) Remember, you did ask for our thoughts! :W:B
Barney