I think the main problem is that if you left foot brake, you might tend to rest your foot on the brake pedal. That can either glaze your brake pads, warp your rotors, or possibly even boil your brake fluid making your brakes totally ineffective. I have never done that, but I did leave the parking brake on once, and after driving about 20-30 miles on the highway, when I went to apply the brakes, the pedal went all the way to the floor without slowing me down much.
Another safety issue if you rest your foot on the brake pedal is that people behind you cannot tell when you are actually braking because your brake lights will be lit all the time. I don't know how many times I have seen people driving down the road with their brake lights on. We razzed my brother about that because we saw his brake lights on as he drove down the block.
I drive both automatic and manual transmissions, and when used to just punching in the clutch, I don't have a feel for left foot braking (I would tend to nail the brakes). Also when competition driving I use my left foot against the wheel well to securely brace myself in my seat while cornering.