Any crack and I condemn it. I won't make it unusable unless its really bad. A crack can lead to poor combustion which will lead to worse combustion. I'd rather err on the side of caution. I can see not doing anything and have it turn bad in a year or two and then come back on the last person who had their hands in it.
This was a big reason I stopped selling Trane. Customers always ask me what is the best furnace to which I can answer with all sincerity that its all about the heat exchanger design. I now only sell brands with tubular heat exchangers, no more clamshell for me ever again. These companies need to sack up and move to a tube design. They aren't perfect, we've all seen cracked tube HX's but they are like a needle in a haystack compared to the daily cracked clamshells. Trane is going to learn the hard way like Carrier.