It is said that the pact a warlock makes with a demon when they first bind them to their will is akin to the pact between master and slave. Warlocks know the depressing truth...
Reduced Initial Cost
If you plan to solo extensively with a Voidwalker or a Succubus, you want this talent. Voidwalkers have so many HP that healing them can be almost prohibitively expensive, and Consume Shadows costs so much mana there's no way he'll be able to use it and still hold aggro for you (going from using Consume Shadows to being in combat; Consume Shadows can't be cast in combat). Succubi. on the other hand, have so little armor this talent is sometimes the only way to keep them alive long enough to be useful when you get adds.