Well, duh. But WTM has a good article on the general role of defense in the Pirates' run prevention, and Jack Wilson's specific role in the Pirates' defense. I'd caution against taking some of the individual UZR/150 numbers too seriously--UZR is a very highly regarded defensive metric, as WTM notes, but it's subject to sample-size fluctuations just like hitting stats are, and those fluctuations can look really strange when you take a 50-game sample and pro-rate it over 150 games. So I wouldn't necessarily put much stock in the idea that, for example, Andy LaRoche's defense for the Pirates last year was 7.7 runs per 150 games better than average.
Despite all that, the picture UZR paints is very clear, and it's corroborated by the Pirates' improvements in defensive efficiency (which measures the percentage of balls in play the defense turns into outs), runs allowed, and observation. WTM uses that picture to stress Wilson's importance to the team. I'm fine with keeping Wilson, both because his defense is a big help and because the trade market for him has been almost nonexistent. If you can trade him for Jair Jurrjens, as Dave Littlefield could have done a couple years ago, that's one thing, but when the biggest name being thrown around is Chin-Lung Hu's, well, you should just hang onto him. In the grand scheme of things, I'm much less concerned with the Pirates' play in 2009 than with the moves they make to set themselves up later, but I do think it's possible that having an ace defensive shortstop can help the Pirates' young pitchers in ways that might conceivably benefit them in the long term.
WTM's post also shows yet again that the Pirates weren't likely to be a successful ballclub with an outfield of Jason Bay, Nate McLouth and Xavier Nady. As helpful as those players were to the 2008 offense, they were huge stumbling blocks for the team's pitchers, and the pitching's apparent giant leap forward this year would not have been possible had the Pirates not replaced Bay and Nady with better defensive players in Nyjer Morgan and Brandon Moss.