Originally Posted by urbanknight

The theory behind this is that when braking, your weight shifts to the front and unloads the rear wheel, so the rear wheel isn't able to do much braking at all if the front is applied anyway. So even when you think you're using both brakes equally, you're really only stopping because of the front brake. Go down a hill and use only the rear brake for a moment and you will see how little it does compared with the front.

That being said, I still brake with both hands most of the time just out of habit. It is true, though, that when you panic and squeeze the life out of both levers, your rear tire skids and you start moving sideways.

edit: Mountain biking is a different animal, though, so don't apply these theories there.