Why can't Jorge Posada get more play at first base and catcher?

Posada has been red hot, and has proven to us that he's not done. But why can't the Yankees use him better. Obviously, he shouldn't catch every day anymore, but I see no reason why he can't catch once or twice a week, and play first base once every two weeks or so. Teixeira and Martin can't play every day. In fact, I think Martin playing so much has something to do with his getting injured. Bad playing time management, if you ask me.