There's nothing wrong with having a rear brake, it's just that it isn't really necessary with a fixed gear. However, a front brake is necessary for emergency hard stopping situations. I happen to ride with only a front brake on the road, and often go without using it for weeks, however, there have been situations when I've needed it to avoid an accident. In wet weather (slippery conditions), you only want to brake in the rear, but with a FG you can do that by backpedalling. Regardless, you still want both brake levers so you have the hood riding position, which is the one I use the most.