Lowell, here you have identified another limiting factor with 2 wheeled transport - a high CG. Yes, I too enjoy seeing how high I can get the back wheel - more so on motorcyles though:)
And the exotic cars - well an Exige is not exactly exotic but then I get your point
At the end of the day, a contact patch 1 or 2 cm wide by 2cm long is going to be influenced in a big way by a 'stone' with dimensions 1x1x1cm - whereas a tyre having a patch perhaps 4 by 4 won't be.
This is what I am talking about - braking in an emergency - this might mean whilst cornering on a less than perfect surface.