Well I was told this by the guy who serviced it and also on other websites I've looked at. If you think about the molecular structue of rubber, it is lots of molecules stranded together in chains. So there are gaps for gas to leak out of. So I would agree with what the guy said. He says that gas only leaks out of rubber and not metal as the metal does not have gaps. He said that the better the aircon system, the less rubber hoses it would contain. I think a fridge system contains a lot more metal! I think you are right in saying then that servicing is preventative because if there is less gas then the compressor has to work harder and therefore there is a higher load which would lead to a premature failure (£800 for new compressor fitted). However my aircon was still cold but it had less than half the gas in due to natural loss. He also said that most dealers don't even bother servicing the aircon on normal service schedules, so the chances are it won't have ever been serviced if serviced by your dealer. I reckon it's well worth servicing it as it could save a lot of money.