This is true… the biggest flaw being everyone in the group gets the DK even if they did not tag the NPC… however they were put in the game to lessen the occurrences of people killing opposite faction quest NPCs… This was a big problem and DKs were in the game before making the quest givers unkillable.
If there are no DKs then quest givers need to be unkillable…
but that is a conversation for Classic +