Rear bias/EBD is very complicated but here are the major points:
Brakes (in general, not P5 specific) benefit in a lot of ways from rear bias in certain situations. The rear bias in a non ABS car is controlled by a pressure sesitive valve that limits rear pressure to a certain gradient after line pressure reaches a certain point. ABS/EBD cars look at both deceleration and wheel slip to control rear bias but the end result is somwhat comprable in either car.
This all means that our cars are rear bias only during light to moderate stops. When you really stomp on the pedal the rears stop getting additional pressure and the fronts are allowed to do most of the work (since during max stopping they carry most of the weight). This maximum stopping is the only time when large brake size and venting are needed so that is why the fronts are bigger than the rears (if we always stopped like pansies the fronts could be small too).
The rear bias at moderate stopping is used for a number of reasons: One, at moderate braking there is not much weight transfer so the rear brakes need to do more work. Two, it helps with a fully loaded car (especially a wagon with weight on the rear). Three, it controls brake dive. Decel produced from any brake will cause some dive, however, some complicated geometry makes it less extreme when rear brakes are used. Fourth, it is intended to make pad wear even on the car, obviously Mazda either screwed this up or decided that reasons 1, 2 and 3 were more important than even wear.
Just to state the obvious, the car can't be rear biased during maximum stops because the weight would transfer forward, the rears would lock and the car would spin (think E-brake).
There's a TON more detail involved here but hopefully this answers the "why" question about our funny pad wear.
Alex
