I think you forget the fact that the Dark has channelers of its own. I estimate that one in three female channelers are Darkfriends, if the Tower Purge percentage holds to be universal. Couple that with the fact that the vast majority of full Asha'man are Darkfriends/possessed (as seen by the Epilogue in LoC, and the Black Tower chapters in ToM), and that means a sizable amount of Dreadlords to help the Trollocs on their merry way.