The Nats believe, and I'm sure they have data we don't have access to to back this up, that Desmond has the "best" range of any SS in the game (as far as I can tell from the statistics available this isn't true, but okay), that his errors are mostly on balls that other SSes don't get to (again, possible, but he makes so many of them that it's definitely cost the team some runs), and that his overall defensive profile is already improving and will improve further with more experience at SS (improbable from where I'm sitting). Now, keep in mind that this is what the FO is saying publicly, so it's possible that internally they're having a very different discussion re: Desmond's ultimate place on the team vs. Espinosa's. Recently Rizzo has been a bit less emphatic about whether Espinosa is really going to be at 2B throughout next year, saying that there would be further evaluation at spring training, etc. But to deduce anything else from that would be pure speculation.
i think riggleman/rizzo are high on him. again the errors were awful this year, but i think he improved a bit and i'm hoping his numbers go down next year. bob/ray would comment some of the plays he made towards the end of the year (eating plays when there was no throw, going for the sure out, etc) as a sign of progress.
and while i think he has good range, i agree he in no way has the best range in the game.