Enabled all year long for me. The only time I turn it off is when I have all of the windows down on a warm/cool day. In Seattle it stays damp all the time and it was like that in North Carolina too in the mornings. Dehumidification is essential.
Think of the AC button as an "enable" function. It allows the AC compressor to be engaged automatically based on demands and control settings. It does not mean it's always engaged.
The AC is pretty smart on BMW's (for years) by using a multi-stage approach for efficiency and demand. Any AC system will not come on when it's really cold--law of physics. However, I believe it can still cycle if you are recirculating the air and it's warm enough in the cabin. If the air is cold enough outside, warming it up is usually more than enough to dry it out. I think front defrost button triggers full fan, full heat, AC, all vents on windshield and full recirculation.
Last edited by snowboardjoe; 12-04-2012 at 01:24 PM.
Reason: Added middle paragraph for more info.