Even if you remove the brake calipers in the back, you will still need the levers for shifting (if you have STI or Campy Ergo). So the only real weight savings will be the rear brake cabling and the rear calipers, not a whole lot of weight. That kind of weight savings isn't worth the problems you may encounter in an emergency braking situation, in my opinion.

By the way, I have only front brakes on my fixie. But my legs help brake the rear wheel.