A spinning wheel has a translational velocity, and a rotational velocity.
Translational velocity is the velocity of the centre of the wheel as shown on the right. Rotational velocity is velocity movement of the outer rim of the wheel.
In normal driving conditions, translational velocity = rotational velocity and there is no wheel slippage.
Now let us consider the case when only the front wheels are involved in breaking, and let us assume say the front wheels completely lock up. Then this will reduce the translational velocity of the entire car, including the rear wheels. Unfortunately the rotational velocity will not decrease as quickly, and this will lead to the rotational velocity being greater than the translational velocity for the rear wheels. This leads to "slippage", which results in friction, and tire wear.
If however the breaking system effects the rear wheels as well, then the car can reduce the rotational velocity of the rear wheels at the same rate as the decrease in translational velocity, and thus reduce the "slippage" and friction that occurs on the rear wheels.
Therefore in conclusion, the breaking system that effects both the front and rear wheels will benefit the rear tires the most in terms of tire wear.