Duke will be eligible for salary arbitration a third and final time this winter, and indications are powerful that management will not tender such an offer, thus casting him into free agency. He currently is making $4.3 million and, because arbitration awards raises based largely on innings, he surely would make more through that process. Another possibility: Management could approach Duke about staying at a lower guaranteed figure.
I'm not sure I can really argue, particularly if the Pirates pick up a rotation option they like better in free agency, much as they did this offseason when they non-tendered Matt Capps and signed Octavio Dotel. It's worth pointing out again, though, that Duke has been royally screwed by the Pirates' defense over the years. He's not a star under any circumstance, obviously, but put him in front of a competent defense, and he's fine.
Unfortunately for Duke, the only remotely competent defense the Pirates have had since 2005 was in 2009. In every other year, they've been in the bottom four in baseball at converting balls in play into outs. In 2009, when they were middle of the pack, Duke's batting average on balls in play dropped 30 points, and he posted a 4.06 ERA. This year, the defense is terrible again, his BABIP is back up, and his ERA is 5.47.
This isn't any kind of love letter to Zach Duke. He's not a great player. In fact, he's a cipher. He lets hitters put balls in play, gets some ground balls, allows a few too many line drives, and generally does little else that's either positive or negative, and so he merely reflects the quality of his defense. Personally, if given the choice between Zach Duke and a pitcher who can take a game into his own hands (and certainly the Pirates have had few of these sorts of pitchers over the past several years, but Oliver Perez showed this kind of game-changing ability in 2004, and James McDonald shows flashes of it now), I'll take a game-changer every time.
While you'd like to have all your pitchers be lions, though, the chameleon does have value. If the Bucs do non-tender Duke, it wouldn't surprise me to see him stick as a fourth starter for a good defensive team. As long as the Pirates continue to field awful defenses, though, he'll probably be of little use to them.