A friend of mine has a fairly new Polo (07 plate) He knows nothing about cars but has always had VW's. He took it for it's first service to the VW garage he bought it from. Amongst other things an oil change should have been undertaken. They managed to drain the oil, put a new sump plug in then not fill up with oil. My mate drove a few miles and the engine seized. Did they accept any responsibility??? No! It took him over a year to get them to accept partial responsibility and pay 50% of the cost of a replacement engine. Needless to say he now uses a specialist or independent.
In my view and in many other peoples view main dealer does not guarantee quality, it is purely for the stamp. A specialist is always happy for your custom and happy to service and maintain older cars without ridiculous labour costs.
If you have a rare, exotic car worth hundreds of thousands with barely any mileage then the stamp is worth it.