People discourage it and it's illegal because stupid people like to ride the brake when they drive with two feet and melt their brakes and have wrecks because they slam on the gas and any other thing you can imagine stupid people doing. As for the rest of us who can drive, it's acceptable and even encouraged for certain types of racing, as long as you're not a moron and don't ride the brake petal. One footed driving was used as a deterrant to keep idiots from riding the brake petal: i.e. it's hard to press on both pedals with one foot unless you're Shaq O'Neal with a size 100 shoe. (then there's heel-and-toe driving for autocross, but if you're a moron who's dumb enough to ride your brakes you shouldn't be allowed on an auto-x track to begin with)