I should clarify that I didn't mean to be prescriptive about future DCA perimeter exemptions, but merely to underscore the negative reputation of IAD insofar as people aren't content with an MDW/HOU/DAL setup juxtaposed with the big hub out in the suburbs to handle everything else when they need that travel beyond. I presume LGA would experience a slight bump in parallel fashion if similar exemptions were allowed, but to a lesser degree given the positioning of NYC airports (e.g., EWR actually being the quickest trip from Lower Manhattan if you're down that way).
In other words, my perception from FT-type chatter is that DCA would see a bunch more beyond-perimeter traffic if those rules were lifted, and you don't have that premium JFK thing going on with pax (though if LGA allowed transcons that would be an interesting experiment to see if UA PS/DL One/Mint/AA FF garnered better yields and traffic than vs. JFK for the same services where the a/c can be kept the same).