On the topic of freewheel equipped bikes, you need to run two brakes as you have to be able to modulate the speed of the rear wheel.

Try descending when it is wet, icy, or on loose surfaces with a single front brake and you will understand.

In most cases, a front brake should be doing all the heavy lifting but there are conditions where a rear brake will make a huge difference... a fixed gear bicycle does allow the rider to modulate the speed of the rear wheel and even skid it and then a front brake is a good idea to maximize stopping power.