The idea is that you could customize both the sound level AND equalization to each speaker. Of course, that doesn't work well with the original head units the cars came with. But with the newer head units you can buy these days, you can set it up so that you only send bass to the rear speakers - which would be, in my mind, a great advantage.

Also, you could cut all bass to the door speakers, which helps as those speakers don't handle bass very well - that is why we do the "biggest bang for the buck" audio modification, we add capacitors in the doors to cut bass to the small tweeter speakers, which REALLY improves their performance.

Also, with six speakers in the car, if you don't drive them using more than two channel amps, the impedance you are driving with the amp is just too low. That is, three speakers in parallel would work out to less than 2 ohms per channel - just too low.