Well to put it simple – certain dungeons arnt rewarding enough for the time it takes to complete them. ARAH is a good example (there are others). Players hate taking inexperienced players with them due to that it only prolongs that time. Players wants to enter the dungeon, finish fast so that they can get their tokens and never do the dungeon again.
Players rage quit or give up as a team due to that the reward at the end just isnt worth spending 2 hours in 1 dungeon. I believe this system only promotes the feeling that dungeons are bad. I only have wow as an mmo to compare and there you could easily spend 3 hours in ICC (mind that I only played private servers) but players stick until end cause they were excited about the drops and that excitement is what I feel the dungeon system is missing.