No, but I have ridden with fg riders. And have ridden other fg bikes myself sometimes offroad. I have over a decade and a half of MTB experience that has taught me what exactly is needed as far as braking goes on trails. Unlike on the road the poor traction conditions on many trails necessitate the ability to use both wheels to stop. Noone no matter how talented can apply even pressure to a rear wheel through the cranks all the time. My experience riding with and talking to very talented fgmtb riders has convinced me that for the vast majority of riders at least it is impossible to apply enough even pressure to ride on steep technical trails without skidding.
skidding the rear wheel to stop but rather unintentionally skidding due to the inability to apply even pressure to the pedals especially at high rpms.

Maybe you don't ride such trails I don't know. Maybe you just ride really slow with a very low gear ratio and walk down alot of stuff. I know for instance muni riders are able to succesfully control their contraptions with only thier pedals. They however walk alot, never go very fast and are geared at 1:1(often with a 24" wheel.) If you ride more like that then I can see how a rear brake would be superfluous.

Since one of the people who rides fg claims to "never touch his front brake" I think I can safely say that my experience is more valuable then some peoples fg experience.

For the last time. I am NOT talking about intentionally skidding the rear wheel but rather unintentional skids that could be avoided if you had a rear brake.