AC systems should be used regularly as the coolant also distributes lubricant around the system which is neccessary to prevent leaks at the seals. See http://www.honestjohn.co.uk/faq/faq.htm?id=14 for more info.
If like mine, the airflow seems very poor and the car is prone to misting up enen with the fan at full speed - the pollen filter is most probably blocked up. Bitch to change (and expensive) but a quick fix is to give it a stap or two with a bamboo stick or similar.!! Its located behind the passenger wiper arm looking in under the bonnet