As a Main DRK myself the best way to give sort out LD is to give it a +% heal increases. Not Damage increase with 0% MP cost.

I mostly use LD but only if the shit is about to hit the fan.

If both my healers are dead it might as well be a wipe anyway.
If I pop LD and I get a +50% damage increases with -100% MP cost then I would use it purely for DPS. Turn off Grit, let the PLD/WAR Tank then use LD for insane damage which would kinda make Grit being used in a way the it shouldn't be used. It's a Tank oh shit skill! Not DPS gain.