I always use both brakes - one exception: When the brake pads of one are used up and I need some more miles to the nnext service.
I fully agree with you that one has better control with the brake than the engine/clutch (and it's no problem to shift down without using engine braking).
But maybe I should mention: I'm not riding a race bike but a sport tourer. So it's NECESSARY for to use both brakes for maximum braking. Therefore there's no discussion to me if I should use both brakes.
"Why not stay in disguise all the time? You know, look like everyone else."
"Because we shouldn't have to."