As a dark knight main, nothing shits me more than having to spend my mp to make my dark mind worth using. I know you'll say "what else will you spend your mana on". Well, when main tanking I'll spend it on:
Bonus enmity on Power so I can siphon more often.
Bonus health drain from soul eater.
Upkeep of Darkside so my dps ain't shocking.

Want to compare dps? Look at dark knight's MT dps.