noglider 

It's a good idea to have two brakes. When you have two caliper brakes, the front one is about twice as effective as the rear. If you have a fixed gear, your legs are as good as a rear caliper brake, so it suffices to have a fixed gear with a front brake. This is true whether you slow with your legs or not. I know a track racer who is also a bike courier, and he recommends using your front brake for braking solely and not using legs. This is a good strategy except in a few emergencies where you should use both.