You'll be glad you have one when:

- You're bombing down a huge hill for your first time and can't backpedal enough to stop. Especially if there's an intersection at the end.

- You're plowing through the city and some ******* runs a red light or a ped jumps in front of you.

Front brakes are the smart thing to have. You can even be discrete with them and mount a bmx-style brake (for drop bars) by the stem or a triathalon brake (for bullhorns) for ultimate coolness.