Dark knights prefer slower SKS because most damage comes from oGCDs instead of GCDs, so normally having more secondary stats like Crit, Det, DH, and even TEN will benefit more. I guess im confused though why you can't take the tome weapon as it would still be a DPS increase, especially the 660 weapon.

Asphodelos DRK BiS was 2.45 for instance and not 2.50. DRKs don't require 0 skillspeed, they just prefer the lowest possible for that patch. I don't see an issue with skillspeed being a potential secondary stat on gear.