Dark Knight's should not use Darkness stance for pulls with 3 or more mobs.....
As per leveling my own Dark Knight, I burn through alot of MP keeping aggro from ranged AOEers and while having a High Damage Melee or Physical DPS in the group. As I have been leveling my 2nd job to 60 which is BLM, I have noticed so many bad Dark Knights in game more focused on acting like they are a DPS role and damage than actually a tank! Darkness is Dark Knights DPS stance while grit is our tank stance.....using it in boss fights is fine since most of the time there are little to no adds.....but during large pulls such as in level 53 dungeon...or the level 60 dungeons....it is so easy for a Dark Knight to lose hate with other geared players which easily wipe a party! Stay in grit stance and remember that Dark Knight is a tank...not dps. Focus on holding aggro and not on what your damage is!