My compressor exploded the other day, when my local VW specialist replaced it they also replaced the dryer unit as it was contaminated with aluminium shards (the engine bay was covered with them as the compressor really did explode). They did this as they said although the compressor comes with a 2 year warranty from VW if they refitted it with a contaminated dryer unit it would no doubt fail within a year and VW could easily argue that the warranty would not be honoured if the compressor was used with a contaminated dryer unit.
I would seriously take this matter up with the dealer, they replaced the compressor last time and it sounds like they didn't change the drtyer unit and it was contaminated (think of it as a filter for your AC). It's a cheap part (about £60) but it's a bugger to fit (2-3 hours), so may well not have been done.