I use a lot of rear brake, I do a lot of descending and I am a heavier rider. I can't think of many times I have locked the rear wheel, it almost never happens.
While it's true the front provides 70% or more of the stopping power, careful use of the rear on a steep descent can aid in your control and help keep the front from getting too hot. Sometimes when I just want to scrub speed I will alternate front to back.
There is a steep grade I do which is twisty and a bit off camber. I use lots of both brakes on that thing.