Hi Jim. What I like about the wedge from the outside is any force or direction of pull from the tie is directed to the inside of the building, post pulling or pushing away from the tie, I feel is more arrested with an external wedge. The slope is cut counter to the half dove tail on the tenon. When you wedge from the inside the slope of the wedge in going in the same angle as the dove tail. Any shrinkage will easily allow the wedge to loosen. I have pounded the crap out of them and then put a fastener in to the wedge, just to make me feel better.
I should get a copy of Will's book, read the review in TFing.