Lets see, If I ride 20 miles every day, my Nexus should last me at least 2 years. Considering the way technology improves there will be likely something I would prefer to have anyway. Although from what I read the Nexus 8spd is supposed to have even better parts and be more efficient than the 7spd ones.
Another way I look at it that what I save in fuel costs by riding my bike when I can will more than pay for a derailleur now and then. I have absolutely no plans to keep any of my bikes for more than 5 or so years anyway although the Swift might be my first bike that has a chance. It may take that long for my Brooks to break-in anyway.
For me my Swift is like a cross between the a Dahon and a BF. You get a fold better more like a Dahon and a ride like a BF. Sort of have your cake and eat it too.