[DRK] Wanna know why Unleashed & Abyssal Drain are both needed?

[Kleys]

Because while Abyssal Drain deal slightly more damages for a slightly higher mana cost...

...Unleashed:
- Doesn't consume your Dark Arts buff (that you'll most likely use for a Carve and Spit)
- Doesn't break your combos, and this is your only GCD having this property.

I've been wondering why people never mentioned that in any of these QQ-DRK-threads so I thought you guys just didn't know.
This is extremely helpful in A2S, where DRK is -spoiler alert- (once again since A1S) the best tank.