only having ~70% of the potential stopping power you used to have.
not being able to trail brake
not being able to hold the rear brake while starting on a hill
if someone slashes your front brake line, you won't be able to stop without unpleasant means
a porcupine could run out of the forest and sprint towards your rear wheel and, if you'd had the rear rotor on, it would have ran into it and bounced back, but because the rotor was off, it ran straight through the wheel and got smashed between your swing arm and spokes and the exploding porcupine could shoot spikes straight into tires causing you to not be able to turn and (because you dont have 100% of your bikes initial stopping power), you wouldn't be able to stop before launching off a massive plateau but landing on your wheels and racking yourself so horribly you stab yourself with a spike from the flat tire in the eyes to take your mind off the man-parts pain.
The choice could go either way.