The Argent Crusade is just one of the neutral factions in WoW, along with the Cenarion Circle, the Shattered Sun Offensive, the Earthen Ring and many others. Chris Metzen spoke about the Argent Crusade and the concept of neutral factions in our Mists of Pandaria press event interview, citing the Argent Crusade as one of the neutral factions that just rang true, whereas the Shattered Sun Offensive simply didn't feel quite as emotional.
But here's the thing -- the Argent Crusade, while being very active in Wrath, is hardly doing anything at all in the Western Plaguelands. There's a fight for Andorhal going on on the Crusade's doorstep, but they aren't stepping in. The Forsaken -- members of the Horde -- are actively seeking out activities that look suspiciously like Scourge activity, even going so far as to recruit the val'kyr, former servants of Arthas.
So ... what gives?