Yes I use the rear brake off road, I use it in parking lots, and I use it in the rain.
I do not use it when slowing down as fast as possible.
As a beginner a person does not know how to feather the rear brake. The front all of the stopping power when emergency braking.
I am talking life and death, there is a car stopped in front of you. You need to stop ASAP, do you think you can modulate the rear brake to not lock up?
Is it better to lock up the rear brake or do nothing to it?
Unless you are on a cruiser or some back heavy bike, I don't think many people can not lock up the rear tire in an emergency stop.
People under estimate the amount of front tire brake that you can do.
As for the percentage of rear brake, during an emergency brake I believe it is 95% front to 5% rear.