Beckham is not playing ss because he was a late addition without a priority spot. That's all. I wouldn't read more into it than that. Greene was considered an iffy defender with a good bat when drafted. Now he's a good defender with an iffy defender. Hard to tell how things will end up when a prospect is so new to pro ball.
Witness to the agony ('76 Sox season tix) and the ecstasy (2005 World Champions!)