its back up its not meant to be always used... but say there was some sort of event that randomly created a large density of humidity in a small area... (some heat source or a body of water) and it was cold... your heater blows hot humid air into the windshield and ur windshield coats over... welp... wish u had the AC on didn't ya if its that dry... you shouldn't need ur defrost on while driving anyway... trust me that AC feature is very usefull... i live in a high plains desert (laramie wyoming) this place is just about as dry as it gets... i came here from texas and my hands cracked and bled... in the winter when it snows and your cleaning off your windshield and you get any water in the defrost system ur screwed... i was coming home from snowboarding... i had a little snow in the car... it melted made the air humid and i almost crashed because my windshield froze over faster than i could melt it...
