A tank's HP dropping to zero and a DRK's MP dropping to zero are not the same thing. HP has multiple factors, how much damage the tank is taking, is the tank dodging mechanics that are avoidable, is the healer healing in cleric stance, is there a down healer, are the DPS standing in stuff they shouldn't, etc, etc, etc.

DRK MP management is completely up to the DRK, and that's it.

And sure nothing ever goes perfect, but isn't that how tanking typically is? We tanks shape the fight and control the flow, regardless of how that flow moves we need to control it, part of this is managing your MP, just in case you have those emergency situations where you need something done right away.