Originally Posted by peripatetic
Don't leave the rear brake on. Bad idea. A rider's feet are directly connected to that rear wheel. You slam on your rear brake, guess what stops turning besides the wheel? Recipe for flying, or at least grinding up some part of your lower leg flesh, depending on your pedal clip combination. A front brake is sufficient, provided it's in good working order.
Never heard this particular warning, and it sounds dicey to me. I run a rear brake on my fixie and have never felt even the slightest hint of a problem using it.
Flying off the bike or grinding flesh...I don't think so.