From what I could tell from video footage, and I could be wrong, but Abyssal Drain's heal is a 200 potency cure per enemy hit, so if you hit 5 enemies with it you get 5 separate 200 potency cures.
With that in mind, in trash packs the AD heal is most likely fine and increasing it's potency could make it too powerful in comparison to the equivalent heal abilities of other tanks.
The problem though is that while fine in trash packs, the heal is pitiful in single-target fights like boss fights, leaving you with a burst self-heal of only 200 potency, 1/6th that of the other tanks.
So really the problem isn't that Abyssal Drain needs it's heal buffed, it's that DRK needs a solution for single-target scenarios.