This only applies in ideal conditions (eg: dry tarmac). Where there is less surface friction (wet or icy surface, dirt or gravel trails, etc) you're going to quickly hit the limits of the tire's traction, so will need both brakes if you want to stop in the shortest possible distance.
Wet roads, cold ice, and dirt still provide enough friction to send you over your handlebars at speed. They just increase the maximum speed rear tires provide any benefit. Near its melding point ice isn’t going to provide enough friction for rear breaks to matter.
So sure there’s a minimal benefit in some very specific conditions, but no they are there for redundancy.
