Something to be said about having an in house average starting SS for cheap who handles the 8 spot rather well (lots of RBI,HR,HBP,BB easy to bunt over, etc). Put your money in getting good hitters in the corners (and 2b) who bat at more important spots in the order.
Short stops touch the ball an awful lot every game. Not just fielding batted balls, but making quick decisions as the relay guy and turning DPs. How many times has he thrown out a runner by a quarter of a step that someone else doesn't get?
Turner has better tools to be a CF. He's hitting well in that position. Nats brass doesn't project him as a SS, and I trust their judgement.
Also WAR accounts well for adjusting for defensive position. But all those stats don't do a good job of valuing plus defense at a postion because defensive stats are generally fluff.