Wonder why Undead Campaign Chapter 1: King Arthas from Warcraft 3 - Frozen Throne felt amazing?
Simply because you got to control Arthas as a scourge minion. You weren't an NPC, were you?
I thought so.
Having a 3rd faction would bring that feeling back in World of Warcraft.

You will no longer have to play Horde, to feel closer to a bad guy replica in the story of World of Warcraft.
You will be part of a Player base Scourge EVIL Faction.
Killing dragons in pink costumes as Horde or Alliance will be a thing of the past. Or at least for those few hardcore PvPers who like to be the total bad ass character.

Trust me. Undead in the Horde isn't sufficient enough.