Chivers,
AirCon does need servicing. A typical Aircon system of the A2's size will be charged with anything between 150 to 250g of Freon (R134a). As a general guide line, most Aircon manufacturers expect there systems to to lose approximately 3g/year (5g/year max) - due to various reasons. Since the initial charge has around 5% tolerance, then in normal circumstances the system should be OK for at least over 3 years (same a warranty strangely enough). Couldn't find reference in my service manual, however the dealer should inadvertantly check the system at every service, as part of the system analysis check. The Aircon panel measures the pressure, hence refridgerant volume in the system - if it is too low, the service engineer will get some feedback/indication.
Steve,
Maersk's comments are correct. One piece of info he didn't add is the mould growth takes place on, or very close to the AC evaporator. When considering the whole system, then nearly all of excess water in the system would be in this region. If the Aircon is on, then very little bacteria can grow. You logic is also correct, however to be honest the amount of water in the ducting is very little in comparison, and therefore never causing any problems.
Kaine
(aka - automotive aircon engineer)