I drove around in my old '92 Integra with no heat shields with no ill effects... until I totalled it.

The main purpose of the shields is to keep the heat out of the engine bay, that's it. The reason they crack is either the basic weakness of cast iron (not suprising) or a poor casting design (unusual for Honda.) Cast iron isn't very strong, I know quite a few cars that have issues with cracking cast iron parts (6G Civics, 1G DSM's, any cast iron turbo manifold, etc)