With ABS if the driver hits the pedal too hard the system stops the wheels locking up. With brake by wire if the driver hits the pedal too hard the wheels still lock up.
Brake by wire should be completely transparent to the drivers if it is working correctly - the brakes should just function like "normal" brakes. In fact, the point of the brake by wire system is to make the rear brakes act like normal brakes, which wouldn't be the case without it due to the ERS.
Even KERS had a significant effect on the rear braking of the car, but it was small enough to ignore the issue by fiddling with brake balance etc. and just work around it. In the 2014 regulations up to ten times more energy can be harvested (from a combination of ERS-H and ERS-K), and the issue can't be ignored any more. Brake by wire is the solution, and it's only a driver aid in the sense that it makes the car act normally rather than randomly. Without it we'd see lock ups all over the place that the drivers could do nothing about and tyres would get wrecked very quickly.