Yeah I agree with pretty much what everyone said as well.
-brakes are cheaper and cheaper to replace/fix than transmissions
-some pressure when stopped incase you get rear-ended
-do more braking earlier and come to a final stop easily and gently
I'm not sure that even on steep descents if I'd use the transmission as a brake with an automatic. I can't help but wonder if the torque converter would be locked or not and if you'd be creating more heat in it if it wasn't. It might not quite be like doing the same thing with a stick.
Just don't ride the brakes all the way down the hill.