I've just come back from a ride on a gravelly bit of downhill road with my touring bike. The rear brake works a treat on it, and for a while there, I was keeping my speed down with the rear only.
Rowan makes a good point here - sometimes traction is questionable, and a rear wheel lockup is easier to control than a front one. I did the gran fondo part of a local road race that includes 18% grades on gravel, both up- and downhill. I didn't even
think about touching my front brake on those downhills!