For how fast Pierre was, he really wasn't that good of a defender. It wasn't just the arm strength, it was his angles to the ball and costly drops. Let's not forget how many runs Viciedo saved by other teams opting not to send their runners home from 2nd on a single. I would expect him to improve just by adding more experience, so he can get used to tracking the ball and anticipating where it will go.
This is a do or die season for Beckham. He is still good enough defensively that at the very worst, he can be a utility man, but we really need to get more out of him.