Isn't NRT pretty severly slot limited? So, a strategy that makes the most of the slots that NW has and its "fifth freedom" rights to take passengers starting their travel at NRT to beyond Asian destinations and at the same time using the correct airplanes to fly point-to-point from some US cities directly to Asia might make sense for the larger DL. I recall reading somewhere that NW was distinctive in having so many fifth freedom rights from NRT. Do UA and AA have similar rights at NRT or other Asian airports?

Detailed explanation of the freedoms from US DOT -- http://ostpxweb.dot.gov/aviation/Data/freedoms.htm