I've had this happen to me in AK too. I'd be in favour of withholding loot until the end of the dungeon in a DF group, or until at least, say, half the timer is up (so as not to penalise groups who try but fail). It's not a clever use of game mechanics (that's when you put together your own group to do it), it's inconveniencing others by either pressuring them into leaving and re-queuing or having to find a replacement. If you want to run these dungeons for the rewards you should actually run them.