Its cheaper to be short when flying
This week Northwest Airlines announced a plan they call Coach Choice. They will hold back some of the aisle seats and the emergency row seats when taking reservations. Then at check in they will charge anyone who would like to sit in those seats an extra $15. Its another way for NWA to get back some of their losses. I've only flown NWA once, it was back before they started charging for the in-flight meal and I thought it was a fabulous meal but I'm not paying extra for food and I'm not going to pay extra for more legroom. United Airlines has a similar program in which they will charge passengers an extra fee at check in if they want to upgrade to unsold seats in a better section of the plane.
On a related note, Financial Freedumb has a link where you can earn free NWA Worldperks miles, maybe you can use the miles to pay for an aisle seat. 200 miles for answering some questions and 2000 if you use the miles in 90 days.