The main reason you see stunters doing left donuts is so they can keep their foot over the brake pedal. You want to have that safety should anything catch. If you watch closely in any stunt video they almost always have the right foot over the brake pedal. Even in stand up wheelies, you'll often see the left foot on the 12 bar and the right foot over the pedal.
A lot of stunters will add a second caliper on the rear and run a line up to the bar on the clutch side. That way they can work the clutch with their second and third fingers and the rear brake the index finger. I had a friend out in AZ that was an awesome stunter, and it really was cool to watch him utilize all these little mods he had fabricated. Here is he and his "crew" doing what they do.
