Usually, you can tell when a driver is using his or her left foot because the brake lights go on and off intermittently for no apparent reason. This is because the power assist feature allows the brakes to be applied with very little pressure.
Braking with the left foot also does harm to the car in two ways. One is that it wears out the breaks prematurely. The other is that it could cause the brake fluid to boil and leave no braking power at all.
Left-foot braking is also a safety issue in that it increases the chance of hitting the wrong pedal in an emergency. This is a major cause of the many of the accidents you see almost daily on the TV news.
Warning: Should you use the brake pedal with your left foot during a driving test, you could fail.
The way I see it, whatever advantages people may see to using their left foot to brake, the risks seem much greater.