Its all a matter of looking to see what the weak link, or limiting factor, is. In the 70s, tires were a limiting factor- they were very skinny, poor rubber compound, poor tread design, and other factors. There was no need to have great brakes if they just locked up the tire. As tires improved, so did brakes. The front of any heavy or high performance bike usually has two disks, two calipers, and often times large calipers with several pistons because the front brake can, and does, do so much of the braking. Rears only have a single disk because they still can't do much of the braking, since weight transfer under braking moves traction to the front tire. Under hard stopping, its very easy to lock the rear wheel even with 1 disk and caliper, so why use a 2nd disk?
Stoppies, besides being entertaining stunts, show that modern front brakes not only have plenty of power to stop the wheel, but that the tires have plenty of traction to handle 100% of the braking. Even an experienced street rider in an emergency situation on a sport bike will be able to do 95-99% of the braking with the front wheel, because the fastest controlled stop will have the front tire doing as much braking as possible while keeping the rear tire on the ground- this would be about 99% front, 1% rear braking.
During normal day to day riding, much of the braking will be closer to 50/50, so its best to use both brakes about equally.
