I've had techs for other companies talk to me during training/certification classes that LOVE the easy sale of telling someone thier heat exchanger is cracked. It's disgusting if you ask me. Why so many people can't do HONEST work these days is beyond me. This guy might be right, but how does he see something that is on the verge of cracking? I'd like to see that myself. He could very easily be showing you things that are perfectly normal and playing it off as something serious. Shut someone's gas off, and they'll pay to be warm.
Get a second opinion. Something sounds fishy to me too.