1) Holding the tiller extension downwind puts it square in the nuts of the guy on the wire. <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />
2) It gives you better rudder control. Steering directly from the tiller crossbar eliminates any flex / slop in the tiller extension.
3) If your're going downwind with a guy on the wire, your butt is parked in the back of the boat. You don't need or want to go anywhere.