Where do you live? Are you in some Eastern area like Florida or Georgia where there's tons of humidity?
I've had lots of cars over the years and have pretty much lived in dry climates on the west coast. I've always kept my AC on at all times even in winter, because it prevents the mildew from accumulating and defrosts faster. The trade-off is that it worsens your fuel economy. AC works fine with heat. I also found that when AC is not on, your windows tend to fog up more with increased humidity inside the car in cold mornings. My wife keeps turning her's off because people are programmed to think you should only use AC in hot weather. It's usually people with older cars that keep the AC off for the entire winter, and then when they need it finally, it blasts that nasty mildew smell. I think most, if not all newer cars, automatically turn on AC when defrosting.
That said, 900 miles seems like something is really wrong... I don't think that should happen and the dealer should fix it for you.