More grip = more strain on the car = more wear + tear.

Why do we need more grip in a spec series?

Also, from what Adam said they help quite a bit under braking. With RA1's the 4-piston calipers don't give "that much" advantage b/c we are still at the limits of the tire. If we go to a better tire then that just makes the Big Brake Kits help even more and then drive the cost of the series up even more.