Well, personally, I'd supplement it with a front brake. Most of the stopping power comes from the front, as the during braking the weight transfers towards the front. This causes the rear wheel to become unloaded, promoting skidding, which is not the quickest way to stop. In emergency situations, the probability of skidding increases. Then there's the extra wear and tear on the tires from skidding. The extra brake decreases your stopping distance and is back-up in case the rear fails. Relying solely on a rear brake is fine, provided you're travelling slowly and plan your braking, but I don't advise it.