Maybe involving the Song. It's simply too convenient that most Tinkers are gathering near Ebou Dar (but I also agree that they will help to cleanse the land of the DO's influence after TG.)

In my view, the real aim of the Trolloc army in TGS, prologue, were actually the Tinkers. The Shadowspawn just coincidentally stumbled into Tylee's troops. Possibly the Tinker's strong belief has in some ways a stabilizing effect on the Pattern. Therefore they are a hindrance for the Shadow.