I can understand people deciding to leave a group which they feel has little or no chance of completing the content. It is their game time after all and they can decline to spend it with whoever they wish. As long as it is done politely I have no real issue with it. The abysmal way in which some people go about it is unwelcome though. I can understand that they may have waited a long time to get into successive groups which prove to be underwhelming at best, and this can lead to a lot of frustration, but rudeness isn't the answer.

More annoying to me is people opening up the group to new arrivals once the instance is complete. Waiting for extended periods only to zone in to hear the victory music and have to re queue is extraordinarily frustrating.