[svandamme]

well it's the lack of range that makes for good practice .
The less travel, the more precise you gotta be.

And if you only got one pedal, you learn to dose your left foot, with the heel to the floor as opposed to the thigh muscles like the stick said.
And obviously it won't teach you pedal dancing, but that's pedal dancing, not left foot braking.
As has been said, left foot braking isn't difficult , it's the pedal dancing that is difficult to figure out.. That's another ability , besides left foot braking.