Since the finger pull on the brake tends to pull the rest of the hand forward, it's hard to add throttle while braking. At the same time, having some grip on the throttle makes downshifting easier. Guess that isn't so important anymore, but when I started riding, 20% of the braking effort came from the front brake, 10% from the rear and the remaining 70% was a combination of compression braking and dragging your feet.
