Thinking about NW passengers (and me) travelling to the USA - why would I take a short flight to DUB, change gauge for the second sector when I can use my local airport MAN and fly a single sector to get where I want. Pre-clearance discounted as it may feature at both airports and, likewise, over and beyond domestically as it amounts to the same.
Price I can hear you shouting but it's still an extra sector ........