IMO, a rear brake is largely used for control and not stopping power. Pure BMX racing, skate park riding and freestyle don't require a brake for power, but are there largely for control. In the dirt flow trail and paved pump track realms, I prefer two brakes where the front is used solely for braking power when needed and the rear solely for control purposes. Normally for trail riding, I'll wear out front brake pads 2x faster than the rear pads and in the flow/pump track realm the rear pads wear out 2x faster than the front pads do. One of my upcoming 26" Pump Track/Flow Trail builds is going to feature a 29er disc fork up front and v-brake rear for the above reasons.
As always, YMMV...