Switching off the air conditioning system to save fuel is a false economy. The ventilation system is designed to be used with the air con on all of the time, regardless of what the weather outside is doing. Not only will this keep the system's seals lubricated and gas-tight, but it will also (as you’ve found) keep your windows clear. Switching the system off for long periods in the winter months could cause the coolant to leak out. It also encourages bacteria to grow in the system, which is then blown into the car when you switch the air con on again.