vs Blood DK / Blood DK: well you can't die they don't have the dps to kill you, and well they may not die either. it's just a waste of time the game drags on and no body goes anywhere. Unless your team have the dps to kill 2 blood DKs but regardless of what comp you play they won't be able to kill you
Blood vs Blood does not take forever as people say it does usually. I usually win Blood v Blood duels with near full HP.
Blood DKs in RBGs : well it just gets out shined by prot warriors, but some could manage to get to high rating in RBGs playing FC blood.
Sure, Prot warriors may have a load of much more mobility then DK's but they usually won't last a long. Blood can usually survive a lot longer.