A long time ago, when I was making a living coaching track racers, a bunch of us got into this argument. As none of us had any physics degrees, we just all got out our road bikes and experimented. The bottom line was that rear brake only was worst by far, front brake alone was very good, and both brakes combined was the clear winner. The technique is/was to hold your weight back as far as possible (abdomen over the saddle) with elbows locked. (Sorry, Cyccommute. It's true that you lose some degree of control with arms locked, but this group of racers ranging from cat. 3 to world champion had no problems with it.) With the body weight held that far back there is still enough weight on the tire for the rear brake to add materially to the process. It also eliminates the possibility of an endo -- although at the limit the front tire is occasionally skidding, which makes some folks nervous.

I think as a general rule of thumb, people who counsel against ever using the front brake are as wrong as people who counsel against ever using the rear one. A competent cyclist knows how to use all the equipment on his bike.