I could see Maurader+Arcanist being a very viable class combo for a Dark Knight job as it would bring both some drain abilities and poison themed DOT's to the table which work well for the DRK theme as well as having a good sized HP pool from MRD which would work well for playing off of the classic Soul Eater ability where DRK could sacrifice HP to do more damage.

If DRK extended from MRD (MRD lvl.30, ACN lvl.15) then the ACN pet would not be an issue since DRK would only really pull from ACN for cross class skills and I doubt that summon would be included for them.