As Sulley points out, IAD has seen a bit of growth since the merger, and looks to see continued service and potentially more growth down the line. For one thing, it serves as the connecting alternative to EWR given crowded NYC airspace and UA's dominance at EWR post-merger (didn't they have to divest a few things to FL or somebody?). All else equal, they might as well route you via IAD vs. EWR. For another, IAD has solid enough traffic between O&D and connections to earn it's keep on the merits, such as UA's ME service. DC is still not the NYC market O&D-wise, but IAD does do enough to keep a prime spot in the network.