Paul:

I believe you are correct in identifing the issue as humidity control. Empirical evidence has show that the excess (over ambient) moisture generated inside the house will alway escape somewhere. Prior to the practice of foaming all the panel joints, this would always occur near the source of moisture (like bathrooms and kitchens). The result was rotted panel skins in an alarmingly short period. Even if all the panels are installed and foamed properly, the moisture will escape near it's source. Thus, the manufacturers are suggesting the heat exchanger to spread the humidity around rather than to provide a source of it. You might be able to conform to the warranty requirements with some sort of local air/humidity control in bathrooms and kitchen, but I don't know if this could be done more cheaply than with a single whole-house system.