Even having missed the past month Desmond is still second in baseball in WAR for short stops with 3.6 to 3.7 for Elvis Andrus. Simmons was having a good year, but Desmond was the best short stop in baseball when he got hurt. Simmons could solve the Braves short stop problems for the future, but he would have to come back on fire to be the reason they catch the Nationals this season.
I think (some) people just see the flaws in the Nats players since they see them so often. We don't have to endure every day of one of Uggla's extended slumps that he has regularly, or the struggles of their lefties against good left handers (.686 OPS versus .825 OPS against righties) - http://www.baseball-reference.com/teams/split.cgi?t=b&team=ATL&year=2012.
Desmond was the best shortstop in baseball this year until he got hurt. Not much arguing with that. Simmons might be great, might not. He wasn't even at the top of their depth chart when the season started so his success was probably even a surprise to the Braves. Pastornicky was supposed to start for them this year.