The rear brake is very important for initial braking, in order to transfer weight to the front, which allows harder braking without locking up the front tire.

When you initially hit the brakes, your braking potential is split around 50:50. At absolute maximum potential braking force, your rear tire is near lifting, so it's around 100:0.

That said, if you only have one brake, it's better to have it on the front than the rear.

Agree that front brake is more important.

However, I like to have both (and suggest you do too!) because:

1. Brakes can fail

2. In slippery and inconsistent conditions, locking up the rear is preferable to the front

If you don't have a rear brake, perhaps listen to people who have front and rear brakes, and thousands of kilometres of riding.

I have literally seen people flip over their handle bars by solely using their front brake going downhill.