That's not quite correct. At 510 potency, Carve and Spit is preferred over Abyssal Drain (150 potency) if you have three or less targets.

The main reason for this decision is because DRK already has a lot of oGCDs during its burst phases (this is also probably why Salted Earth is on an awkward 90 second recast). It would be nice, however, if they had an appropriately sized self-heal on Carve and Spit for single target scenarios.