There are two players who have won trophies at RFK Stadium. One of them in Dwayne De Rosario, who led the Houston Dynamo to the 2008 MLS Cup crown in a Final played on East Capitol Street. The other one is James Riley, who lifted the 2009 U.S. Open Cup along the Anacostia with the expansion Seattle Sounders (a game I had the misfortune of attending). (Unrelated to anything, and apropos of nothing, Alex Prus refereed both of those games, the two most recent cup finals to be played at RFK.)
But it's not the fact that Riley helped invent the Open Cup that convinced Ben Olsen to bring him in. Quite simply, he was a quality piece available for a reasonable cost. It doesn't matter that he plays a position at which we already had reasonable depth, acquiring James Riley would make us better. Even if he doesn't fight past Chris Korb or Daniel Woolard for a starting spot, he immediately brings competition for playing time to both fullback positions. That makes the incumbents work harder to keep their spots, and that will come through in the game.
Rather than rehash the whole argument, I'll just point you to ChestRockwell's piece on Riley's acquisition by United from earlier this month and ask you readers how much time you see Riley getting this year. Think he'll earn his way into the First XI - as he has at every other team he's played for - or will he be left to spot starts and Open Cup duty?