From Sheldon Brown:
Maximum Deceleration--Emergency Stops
The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear wheel cannot contribute to stopping power, since it has no traction.
Using both brakes together can cause "fishtailing." If the rear wheel skids while braking force is also being applied to the front, the rear of the bike will tend to swing past the front, since the front is applying a greater decelerating force than the rear. Once the rear tire starts to skid, it can move sideways as easily as forward.
Before I knew better, I locked up the rear in an emergency stop and started the fishtailing. Letting off the back brake stopped it. But I was lucky I didn't crash.
Back brakes are good for alternating wheels on long downhills, so the rims don't get too hot, and are fine for normal easy braking. And I use the back brake in slippery conditions.