However, on a scooter, the rear brake is A LOT more efficient then on a normal motorcycle,
especially on the butt-heavy scoots.
The Downtown can stop rather fast using rear only since the rear wheel has a lot of load on
it, but using both will make the stop happen a lot faster (and safer).
While it is true that rear brake might feel more efficient on some scoots than on some bikes (Piaggio NRG 50 has unbelieavably good rear (and front) brake for such a small scoot), considering that most scooter drivers are newbies and/or inexperienced and have no idea how 2-wheeled vehicle behaves under extremes, braking with both brakes is highly adviseable, maintaining most pressure on the front brakes.
The amount of stopping force that can be produced by front wheel can by no means be compared with amount of braking force produced by rear wheel.
Furthermore, under extremes, balanced front and rear wheel braking is the only way to maintain maximum stopping force while keeping some resemblence of vehicle stability.
Personally, the more I drive scoots, the more I am worried about wheels locking because bigger scoots have more than ample amount of stopping power that tends to spill over into locked wheels. When I drive my People with dual front discs, and have to stop suddenly, I have to take great care not to lock the wheels and the pressure that has to be kept on the levers is relatively slight to come to locking point compared to some scoots from just 10 years ago.
Braking technique may greatly vary depending on driving style, road type, bike load and bike type. For example, my Kawasaki had too soft front telescopes prone to excessive dive during aggressive curve approach. For example, if you approached the bend at 100-110 mph (and there is a number of such situations on the road where my group used to go to ride), but could only negotiate the bend at 60-65 mph, you could be faster if you depressed the front brake while increasing the pressure on the rear wheel in the final braking stage before opening the throttle fully after the apex. This is contrary to most braking tips you will hear but suited the dynamics of that particular bike nicely - it would probably send you rubber side up on most other bikes including sports bikes. However, this kind of brake usage is an exception and should not be used on the regular roads by regular drivers anyway.