A rear brake on a single speed is good, not riding one on a fixed gear is pretty goddamned stupid if you ask me:

- better braking power in the rain: since you are distributing the resistance over two tires you can stop more quickly without losing traction.

- if you are braking hard with the front brake and ever lose traction (skid) with the front wheel, you can release the front and tighten the rear to straighten the bike out and keep you from falling on your ass. This scenario can play out very easily on bumpy ill-maintained urban roads if you are not moving slowly (you are braking hard, bump causes front wheel to leave ground, it locks up and you start skidding with the front wheel). I probably do this once a week since I have some very bumpy roads in my commute. I'd have to slow way the hell down on those roads or eat it very frequently if I did not have a rear brake.

- unlikely but: if your front brake fails and you do not have a rear brake, you are up **** creek if you need to stop suddenly ... very unlikely that you're gonna have the time to reach around and foot-stop the rear wheel when you're careening towards a stopped car/pedestrian that has suddenly appeared in your path.

Personally, I think riding SS without a rear brake is intensely stupid ... It makes sense on a fixed gear because you can backpedal, but not on a SS. I see people coasting without a rear brake occasionally, I usually assume that they are pretending to ride fixed ... which is pretty lame if you ask me.