Why don't the Cubs start Zambrano at first when he's not pitching?

He could probably hit .230 or so with 15 home runs. And Pena can't hit .200, and he only hit 18 a year ago. It's not like he plays a gold glove first base either. He got one in '08, but that was because of fielding %, not great plays. Pena can be a powerful pinch hitter and first baseman when Zambrano's pitching