When I took the motorcycle safety course back in 1988 they absolutely taught us to use all four fingers on the brake lever. In emergencies you want to be able to provide a lot of quick progressive pressure, and four fingers are better than two for that. Plus it prevents the possibility of getting fingers pinched between the lever and grip which could hinder applying full pressure. There isn't always time to think things out in a panic stop, so minimizing the chances of an accidental pinch is good.Four fingers is not proper technique. Preference or not, off road or on road.
So why do people upgrade their bikes brakes then?Four fingers stronger than two! Not all roads are perfectly flat and dry without some sort of dirt, oil, sand or cracks filled by tar. Four finger grip will apply more force and thus be more dangerous in emergency situations.