A common refrain this off-season, as the LaRoche drama has dragged on is that the Nats need more left-handed power. Last season the Nats two main left-handed batters where Harper and LaRoche. With Harper moving from second to fourth in the line-up as expected, and Span at the top of the line-up the Nats will have the same number of left-handed batters, but they will lack the power that LaRoche brought. As has been pointed out before, Morse is just as effective vs. right handers hitting .292/.343/.487 for his career compared to .274/.348/.495 for LaRoche. With that little of a difference between the two the Nats don't have much need for more left handed power.
A league average left-handed batter hit .260/.332/.416 against right-handed pitching in 2012. Several of the Nats middle of the order bats are better than this. Ryan Zimmerman for his career is hitting .273/.333/.478 against right handers, Jayson Werth .260/.352/.436, and in his breakout season of 2012 Ian Desmond hit .289/.336/.492. All of them hit left handers better, but that isn't the point. The Nats have right-handed batters that can approximate the production of a league average left-hander.
The worry with this is that when it gets late in a game the Nats will be an easy team for a manager to play match-ups with as the batting order will feature several right handers in a row, but with those right handers likely to be Werth andZimmerman there is little difference between their production against right-handed pitching as the MLB average left-handed batter.
If Adam LaRoche does indeed take his talents elsewhere then the Nats are likely to bat Harper fourth. It is hard to project what Harper will do, but two projections ZiPS and Bill James have him hitting close to his 2012 line of .270/.340/.477. That is understandable as projections are based on career averages with more recent seasons weighted heavier and accounting for decline. As Harper has only played one season it is a bit hard to project what he will do, but compare his 2012 batting line to LaRoche's of .271/.343/.510. The only big difference is in the slugging and it wouldn't be much of a surprise if at the age of 20 Harper hits with more power in 2013.
Combine any advance in Harper's power with Morse, Zimmerman, Desmond and Werth's splits against right-handed pitching and the Nats need for a left-handed power bat simply based on handedness seems silly. The Nats line-up is full of right-handed hitters, but those hitters can hit right-handed pitchers at or above the ability of an average left hander. The Nats have a good line-up with or without LaRoche, and if a manager wants to bring in a lesser right-handed pitcher to deal with Werth and Zimmerman late in a game instead of a better left-handed pitcher it can be certain that Davey Johnson would welcome the mistake.