I don't dispute that Martinez is on the bottom-rung of right field defenders, but I question the premise that this outweighs his batting prowess. I watched several years of Matt Holliday in left field making bad plays, yet he was in the line-up every day. Why? Because he could hit.
Holiday was an average defender during his prime years. His outstanding offensive production made him a top tier player and his defense was nowhere near as bad as JMart (at comparable ages).
I would argue Holliday remained in the lineup even after he became a defensive liability because, yes, he could still hit. But also because he was the highest paid player on the team and a clubhouse leader. Just like JMart, he was frequently removed during late innings for a better defender.