Why haven't they just taken over already, especially at this stage of the game? I get that they might want to turn another few to the Shadow but why risk it? Shield the few who are worthwhile until they can be turned and kill the rest. No point in messing about with the Last Battle approaching so quickly... By the way, the trick that Rand used to cut off Asmodean from the Dark One: could it be used on channelers who have been turned?