That does bring up something I forgot to mention on Dark Knights. I think they make more sense on Marauders due to them having a lot more health normally. If the HP cost thing is a consistent thing for the DRK design, it needs the health to pull it off without relying on draining HP pretty often. Though, I wish there was a way to make it optional more consistently, but barring including it as part of a combo I can't think of how to do it.