If you only amplify the rears, all your sound focus is going to come from the rear.
When you go to see live music you don't turn your back on the source of the music!!
How many cars that compete in sound quality competitions run amplified speakers at the rear, with non-amplified fronts? None, because it is nothing like a true reproduction of how you listen to music from its source.
Amplified 6x9's are a good start on getting a loud system in the car, but they're not synonomous with quality, because a system heavily biased towards the rear goes against faithful sound reproduction.