Am I missing something about DRK?

[Hundred]

You basically never do more then 1 power slash combos on a single enemy in this game for dungeons, and like 3 at most if that for most bosses and often won't use it when dealing with mobs given unleash will cover that. Basically all Drk does is Soul eater and delirium combos which recover mana. It's literally 1 syphon every 20s to pay for Darkside which is lamen terms is "if all you do is maintain Delirium, you will pay for Darkside".

Once aggro is established only Dark arts and DP are mana taxes. Mana for Dark arts is provided at the same rate as Warriors. 8 GCD's. Only difference is that Darks are frontloaded with their "stacks" (mana) where as Warriors start from 0.

Simply plug in Blood Price then throw in Sole Survivor opportunities and Carve & Spit if you get overzealous.

Really simplified rotation:
Power Slash, 3-4x of Delirium/Soul eater combo, Throwing Dark arts and Blood Price as you see fit. Maintain Salted Earth on your target and Scourge. Though basically when you look at your party list if you're not going to lose aggro, power slash doesnt exist. It provides no sustain unlike Soul eater and DA soul eater and it's less damage then Delirium its sole purpose is aggro which you already have.