I know its cliche, but in air-to-air training, pilots have to make decisions in fractions of a second. Sometimes the amount of time for a decision isn't even enough to allow the pilot to pull the ejection handle.
Wrongwayric is right about stress fractures. My profession is non-destructive testing. The air force does allow its planes to have cracks in the wings, as long as they are under a certain length. The Navy however, does not.