Well the main issue is rust, but heat also. The manifold is a big (1 piece) straight 6 unit, so there isn't much room for expanding or contracting under extreme change of temperature. This is what causes cracking in the first place.
When we're overworking it and it's extremely hot, often cold water will splash up, hit the headers, and change the temp drastically. This could contribute to cracking too couldn't it? I was thinking that maybe a ceramic coating could keep temp. down and help keep the cracking from happening again...
but then again that's just my way of thinking