The reason for the perimeter rule dates many decades when your average narrowbody aircraft could not fly transcons. For a DC-10 or other type of aircraft to takeoff from DCA, the engines would have been running at full throttle. With those older planes, the noise in the surrounding communities would have been very high, so they put the perimeter rule in place to keep this from happening. Even today, an A320 with a full load of passengers, cargo and fuel needs quite a bit of thrust to takeoff on DCA's relatively short runways, but A320s/737NGs are quiet by comparison to the planes previously used on transcons. By comparison, I don't think an A321 flying DCA-west coast could even get off the ground at DCA, though a pilot would know better.