That's only relevant in situation where MP are never restrained, which are pretty rare due to the low MP return on BP now.
In usual situation, blowing MP on DA DP will restrain your AOE efficiency and at the same time, reduce the twin ressource gain on BP due to the blind, and might avoid your next TBN to terminate and waste it's cost.
The actual gameplay of DRK in dungeon is scaling too much out of the number of ennemies. Like every job "the more they are, the more damage I do by hitting more ennemies", but at the same time "the more they are, the more ressources I get from beeing hit" and since ressources are starved tight, the benefit of numerous ennemies is even more relevant.
At the same time, using TBN with too few ennemies and too much gear make it a waste, and either you don't use it with 4 or less ennemies alive, either you even avoid popping other cooldown because the EH you will gain from it might waste TBN altogether.
TL,DR : The DRK is too much dependant from the number of mobs presents, making small pulls, undergeared pulls, or non-confident healer pulls a big boredom of spamming siphon strike 2 part combo, and avoiding our new cooldown like plague.