New to automatics

If your h-brake can *easily* hold the car on a steep incline, and I mean the ones at maybe 10-15 degrees that you get between floors in some multi-storey car parks, I would say it's exceptional. Most cars I've owned would need the buttistance of the footbrake when the h-brake is applied to hold the car on such a steep slope.

Optimum for a manual, but in an auto you have a spare foot. So why not take advantage of that fact.

One reason is that it's physically easier. It takes less effort to use the footbrake. Another is that it's much simpler to put your left foot on the brake rather than reach down to apply the h-brake. It makes ergonomic sense. Another point is that some cars have h-brakes that are not easily applied. Especially if it is not close to the side of the seat.

If I'm on an incline, I'm not likely

My regular car, an auto BMW E39 has an very poor h-brake, but that's not why I use my left foot. This is my 4th auto, and I've done it on all of them. You shouldn't knock it until you try it. :-) Mike.