JFK and IAD are both (IME) more difficult to get to and take more time to get through security and to your gate. DCA is great like that- I leave my house 1 hr before a flight, get there with 45 left, and am through security and to my gate before boarding starts. For IAD, I have to drive (in lovely nova traffic) and park, or take the metro and a bus.

I agree that the perimeter rule is silly and outdated. AS flies two cross country flights a day. However, UA fights to keep it in place so they can keep their operations centered at IAD (and for JFK/LGA, DL/AA seem to do the same)