Unleash is a moderately low cost effective way to generate initial aggro. Abyssal Drain is a High cost but effective way to recover HP against many enemies. This is the purpose of the two moves and how they're supposed to be used. Do they both build aggro? Sure....but abyssal drain is used more for the HP recovery and not so much for the aggro. CAN you use abyssal drain to start a pull? Sure...most don't, however, because most would rather save the difference in MP to use to DA boost Soul Eater or another ability. When running Dark Knight, you don't really wanna use more mp than what you really need to use since the job as a whole revolves around MP management. You also shouldn't be using blood price until after you've already established aggro on your target(s) since if you use it before you pull, you're wasting several seconds on BP's timer just to get all the target(s) in order.