Hex + Damballa spawns new zombies, but it also makes it near impossible to keep them up for clearing mobs because as soon as the new ones spawn, they immediately blow up again even if all the enemies are dead.
If it was channeled like Possession, you could stop the channel after the enemies are dead and keep more zombies per encounter.
As the skill works right now, any new zombies that spawn from the passive or hex will just immediately get blown up again.