Jim,
Flying stunt with a 1/2a, while not super difficult, is a challenge. This is primarily due to most of us pulling out the 1/2A stuff a week (or day!) of the contest. These engines are temperamental, sensitive to small amounts of contamination, and make their power at high rpms. At these rpms, the props are pretty inefficient. However prop size is basically the only "universal" denominator of 1/2a these days.
Allowing a much more efficient 7" prop means that electric becomes almost a no-brainer. I think what most of us are concerned about is that 1/2a is fun, and I for one, would hate to see it turn into a turn-key electric only event.