I don't believe it's 'wrong', however I would say that if I need to decrease speed (especially in a dangerous condition), I wouldn't want anything hovering over the accelerator. And using the left foot on the brake you either have to plant your right foot on the floorboard or hover it over the accelerator.
I can recall a time where the brake lines on my '89 Bronco split and the brake pedal went to the floor and I ended up having to hold the steering wheel and plant my foot all the way down to even get any brakegrasp after pumping.
Both legs were pushing forward with more force than I've ever had to use. My left foot was on the rest designed for it and my right foot was on the brake. I had no reason to press hard on the rest, but trying to stop a multi-ton truck, it was just a reaction to press both legs to prepare for impact. If I was a left "foot-braker", I truly believe even with planting my foot on the floorboard in front of the accelerator, as there's not a place designed for the right foot as there is for the left, it would have ended badly as the foot would have slid into the accelerator with the force being applied out of instinct/reaction. There is no other place for your right foot to go but the accelerator if your left foot is covering the brake, unless you can cross your legs or quickly manage to re-train yourself then switch feet, which kinda negates any claimed 'better reaction time', to brake with your right in an extreme situation.
/what gets under my skin more is people inching forward at a red light being the lead car, or a car or few behind the lead car.
//Where are you going? The light hasn't changed, and I don't think that guys is going to run the red!