This 'service' seems to be one that the stealers add on, recommending that it's done every two years. You'll all see however that there's no such recommendation or requirement in the VW Service Schedule, or action in the service book (if you have one), to have this done. It's just another discretionary invoiceable opportunity for the stealers! All they do is vac it out and put the same refrigerant back in again, topping up if any has been lost. As everyone else has said if it's not broke.....
Also the comments above to use the A/C regularly to stop seals drying out etc... I have another German car, and that runs the A/C compressor for 30 seconds or so on every start-up, regardless of outside or set temperature. I guess that's along the lines of regular use of the system and keeping the refrigerant circulating..