So far, since i got my Messenger, i haven't used the rear brake all that much.
It's not too big a deal to back pedal to stop my bike, and i got used to using just the front brake on my last conversion and it was a single speed.
of course if these fail, you're not stuck without ANY options, you can always press your foot of the wheel to get yourself to stop.
When i decide to change handle bars, it'll have only one brake on it, and since i check my bike when i get home, if there are any problems with it, i'll know before i find out the hard way.