I'm not sure, either. I just used Ellsbury as an example of someone who started out shaky on breaks and routes, and ended up being a plus, maybe thanks to his speed, but Duran has that, too.
So far, I agree, Duran has not looked good on D, but he barely has over 200 innings out there. I'm willing to cut him some slack.
He should only play LF until he shows he can do more.