Having ridden and raced bicycles and motorcycles, I know that three-fourths or more of the stopping power comes from the front brake, but the rear brake contributes to stopping, and I always use both. Nothing like grabbing a handful of front brake and then having the front tire hit a slippery spot on the road, locking up the front wheel. This can result in interesting acrobatic movements, or instant contact with the pavement if you are in the midst of a turn.