War Machines aren't the target for Killing Blow Archers - Heavy Cavalry/Tanking Infantry are,with their 3+ or better saves, Goblins will struggle to counter them.
Exactly as Vaz says. The Harbingers are to give the goblins a reliable counter to a unit of Chaos Knights, Black Knights, etc. The Fungus Troll is obviously capable of this role too, but then it isn't exactly reliable!
I think next I am going to start work on the Magic Lore, but I may change my mind if I fancy a character or two first.