It is a higher potency WS than Souleater when buffed - when unbuffed it's only half the MP return of a Dark Arts.

IMO the only time not buffing it is valued is AoE situations where the extra MP covers the cost of Dark Passenger, and you can Dark Arts buff Passenger to 250 potency AoE per mob (3+ minimum).

Trading a DA buffed CnS for a DA buffed Souleater doesn't seem right to me.