Subbing DRK for a lvl 37 spell is rediculous. Absorb-AGI takes AGI from the mob and gives it to you, but subbing DRK as a RNG just for that seems stupid to me. There are much much better subs out there that do way more for a RNG than a simple spell.
I don't mean to sound harsh or anything, but to me it doesn't make any sense. NIN/WAR/SAM even THF give more usefulness to RNG than a DRK would.