You should always use both brakes. … Jumping on only one brake in an emergency has the potential of making the situation worse than not braking at all.
Wrong, and very bad advice!
Maximum Deceleration--Emergency Stops
The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear wheel cannot contribute to stopping power, since it has no traction.
http://sheldonbrown.com/brakturn.html#frontorrear
For most upright bikes on clean, dry, level pavement: The rear brake is for minor speed adjustments (and for use in the unlikely event of a front brake failure). The front brake is for stopping.
Personally, nearly all of my urban stops are "stoppies," with the rear wheel a few inches off the ground. Emergency stops should be well practiced before they are actually needed. The ability to modulate my braking while balancing on my front wheel under highly stressful conditions has saved me from several accidents over the years. I only had to go over the bar and headbutt one car before I learned that there's a difference between understanding the physics behind emergency stopping and actually being able to safely perform an emergency stop when the situation requires it and the adrenaline is pumping.
With the possible exception of route planning, there is no more important skill for urban cyclists than the stoppie.