I think Brian hit the nail on the head. The key to safety is distance. The current AMA recommendations for field size are way too small for the kinds of planes and the way they are flown these days. (My club just built a field exactly to the Pub 706 specs, and 45 feet between the runway and the pits is just too close for (my) comfort.)

High speed jets are fine out on the desert with a few people watching, but putting on a jet show with a crowd of 100s of people just 65 feet from the runway is inviting disaster. Same thing for 3D 33% models and helicopters.

Flame me if you will, but I'm comfortable with folks 65 feet from a trainer with a 10" prop on it.