J.J. Hardy is hitting poorly this year. This does not mean that J.J. Hardy is a bad hitter.
In 2007, Hardy swung at 22% of pitches outside the zone. In 2008 he swung at 24% of pitches outside the zone. In 2009 he has swung at 22% of pitches outside of the zone. So his plate discipline hasn't really changed.
He has swung at a slightly smaller amount of pitches overall than he did in the previous two years. The last three years he's swung at 39, 42, 38 percent of total pitches. In addition he's swung at about 5% fewer pitches in the zone, and making contact with about the exact same amount of pitches that he swings at in the zone.
One noticeable difference I see is the fact that though Hardy has not really swung at more pitches outside the zone, he has made contact with fewer of those pitches outside the zone. As a result his overall rate of making contact with pitches has dropped a bit, from about 83% to about 80%.
The batted ball trends for Hardy are nothing out of the ordinary. A quick look could tell you that his line drive rate has dropped 3 consecutive years, but it's a poor method to only look at a classification like that to tell you everything about batted balls. His trends have switched back to how they were in 2007-- more flyballs, fewer groundballs. His HR/FB rate has dipped, and that should see a rebound-- in general, a hitter will see a pretty constant rate of flyballs turn into homers over the course of a season.
There are a few differences in Hardy's skillset from this year and last, but nothing to indicate that he would see a drop in average on balls in play from the .280-.290 range down to .262. From here on out, we should expect him to be something much closer to his career rates. Accordingly, it might be wise to hang on to him until the end of the year at least and I would imagine his trade value would rebound.
Teams are starting to value defense properly, and a league-average type hitter like Hardy who can play better than +10 defense at short (as he has this year) has a tremendous amount of value, even for only one year. Hardy isn't a poor hitting shortstop with limited trade value. Let's keep his season in perspective. He's having a down year, and it's being compouned by some bad luck.