The received idea is tenacious; we wouldn't use the rear brake in MotoGP, if only because the rear wheel is often in the air during hard braking.
So why put a rear brake?
First of all because it is made obligatory by the regulations, but above all, because most pilots use it, whether Marc Marquez when accelerating and for 80% of a lap, Andrea Dovizioso to the point of having a thumb-controlled rear brake fitted for right turns, or even Scott Redding as seen in this video.
Interesting detail, we notice that extending your leg when braking does not prevent you from braking, just afterwards...
