Why I will only use disc brakes:
no rim wear so no rim replacement every 6 months
consistent braking no matter what the weather and roads/trails are like
masses of power and great modulation
VERY simple to setup, adjust and use
no hassle changing/removing wheels (open skewer, remove wheel, replace wheel close skewer - no F. about with cables or brake release)
no rim contact so no worries with out of true wheels
no grabbing or pads wearing so fast you have to replace every 2-3 weeks (especially with wet, gritted roads or muddy trails)
MUCH easier than cantilevers to install, adjust and maintain
disc brakes only