Most of the rearbrakes I've designed only weight 33 - 50% of an OEM rearbrake, so they shave from 0.7 to 1.1 kg , which is 1.5 - 2.4 lbs.
Of course the stopping Power if my ( mostly hidden and close to invisible ) rearbrakes is not as good as OEM, but they're designed anyway as Racing - Brakes, for those Riders who use the rearbrake seldom or never .

But here we go:
a light rearbrake helps a lot to let the Supension work better, because it's UNSPRUNG Mass.
Just Imagine the spring of your Shock is an Engine of a Bike, which has to accelarate the Swingarm at rebound - so the heavier the Bike ( here Swingarm w/Brake,Wheel)is the more time it takes to accelerate it and to follow changes in the Asphalt.