This is an extremely important point. In an emergency stop you need to steer as well as brake, and with both feet up in the air you cannot brace yourself for emergency steering. You may begin your braking a faction of a second sooner, but you will not be able to steer as well, and with your right foot in the air over the accelerator, you probably won't be able to brake as hard as you would if your other foot is resting on a firm support.
But this applies to manual cars as well, both feet are doing something in an emergency and you have to balance yourself without your legs.
It may be a preference but its a really really poor driving habit. Bet you don't use your turn signals either. What do you do if you have to drive a stick car all of a sudden? Your then forced to ride the brake or hover your foot over the brake. It becomes very dangerous especially when you get real old and get confused. That how really old people drive because there either too lazy to move there right foot or its painful to move the right foot. Down here in Gods Waiting Room there's daily news reports of some old coot driving through the front of a store. Besides its really uncomfortable unless you rest your left foot on the brake pedal. A normal driver has his left leg planted for support which comes in real handy for an emergency stop. Left footer's are easy to spot when your driving with them because it's hard to brake smooth. Very jerky ride.
Au contraire! In a manual, if you have to stop, you press the clutch all the way to the floor, and your left foot is braced as firmly as if it were on the floor. But if you are braking with your left foot you habitually have both feet in the air and you probably won't put your right foot down when you are braking.
Au contraire! If you do it correctly then you do not press the clutch until the engine is going to stall. If you de-clutch as a means to support your body, at speed, you will increase the likelehoood of a skid. At least 99% of the cars I drive are manual, the Prius is the only automatic I drive regularly though I do drive a lot of them in the course of repairs. Each to his own, when done correctly you cannot tell a left foot breaker from a right foot without looking, people who jerk their way to a stop with the left foot are not doing it correctly. I think that normally I will use whichever foot is convenient with no preference either way, it depends how long I have been driving, which leg is tired or a whole host of other things. I will only always use the left foot if there is something wrong with the car. Anyway, enough from me. It's just my way and the 40 years I have been doing it like that. To old to change and to old to bother.
Starting up steep hills. The 'creep' feature is not strong enough to prevent the car rolling backward down hill after you release the brake (right foot) and move to the gas. Not much different than using the parking brake in a MT car starting on a steep hill, just that we can use the 'real' brakes with our generally unused left foot.