Because of the logjam in the outfield, Willie Harris's chances of seeing playing time in the outfield is slim. He's currently listed third on the Nationals depth chart in center and way down at fourth in left behind Wily Mo Painful (c'mon now, Ladson, that ain't right...).
It's true that he can play second. And in a real pinch, he can play a little third or short. But he really can't make the throws from there. He's only started six games ever on the left side of the infield, three of those for the Nationals with their ridiculous 2008 roster snafus. Harris really isn't a utility infielder.
The reality is that most of Harris's value is in his outfield defense. Take a look. Averaging out five prominent projection systems (CHONE, Bill James, ZIPs, Oliver, and Marcel), Harris is expected to post a .325 wOBA in 2009. The wOBA stat is a composite metric measuring overall offensive value, and it is scaled so that the league average wOBA is the same as league average OBP. Average OBP in the NL last year was .331, so Harris is expected to be a touch below average at the plate.
Let's assume that he was a full-time left fielder. For his career, his average UZR/150 is 18.5, meaning that he prevents 18.5 runs better than the average LF over the course of a full season. A .325 wOBA and a 18.5 UZR/150 gives you a 2.30 wins above replacement LF. In CF, his career UZR/150 is 10.8, and therefore as a full-time CF he's a 2.51 wins above replacement player.
But at 2B, his career UZR/150 is just 1.4, making him a 1.72 wins above replacement player. In other words, by playing him in the OF, you increase his value to the team by 0.6 to 0.8 wins over the course of a full season. That might not seem like a lot, but when you're thinking about optimizing player value with decisions like this it is. Think of it this way. Tom Tango has shown that using the perfect batting order is worth 5-15 runs over the course of a season, which translates to about 0.5-1.5 wins. And how much time is spent fretting over batting orders?
Of course, he's not going to play either position full time, but the point is clear that using Harris primarily as an infielder significantly reduces his value to the team. Among all the other reasons to clear out one of the Nationals extra outfielders (Willingham), add that we can raise Willie Harris's value by using him as the fith outfielder he ought to be.