With any engine with aluminum heads its generally not good to leave the plugs past 50k miles. Carbon deposits can harden and build up at the end of the plug. When you remove the plug the deposits can wipe out the threads. Also, after a while the anti-seize (if any) will burn off letting the two types of metal (aluminum head + steel threads of the spark plug) to start bonding to each other. That will wipe out the threads as well.

That being said, I feel the 30k interval is a little much. I still follow it though. In the two times I have changed out plugs all of the plugs looked fine and still had proper gap. The OEM plugs don't cost that much and are easy to change so its just like extra insurance.