Why play an MMO? because you are playing with many other people who have different playing styles and different levels of ability and understanding. That should be the assumption going into a game like this. If everyone was a perfect player it would be pretty boring because there would be no real challenge. Nobodies life is going to be effected if they fail to clear a dungeon in ARR. It's meant to be fun. I have never been in a dungeon with random players yet, where anyone asked anyone else what their playing style and abilities are before forging ahead. Maybe that would be a good idea.

Or play single player games. At least then you always get what you expect.