I normally use both brakes, with a bit of front bias. If I'm braking hard enough that my rear wheel starts to skid, I reduce the rear braking force. My bike usually has stuff on the rear rack, and I sit quite upright, so you can use a little more rear brake than you would be able to when leaning forward on a road bike.

There's nothing wrong with using the rear brake for a controlled, planned stop, but not being used to using the front brake (possibly through fear of going over the handlebars) can lead to going over the handlebars when you can't stop on rear brake alone during a panic stop and grab a fistful of front- essentially it's a self-fulfilling prophecy. Correct use of the front brake will give the quickest stop. Both brakes do have their uses, but the front is by far more useful. If I had to choose only one brake, it would be the front one.