Zach Duke and outfield defense

Once upon a time, in the early days of WHYGAVS, there was a pretty heated debate between myself and some of the commenters about Jason Bay's value on defense. This was before really advanced defensive metrics were widely available on the internet and I was arguing that while Bay had a weak arm and wasn't the fastest cat out in left, that he had generally good positioning and ran good routes and that made him a good outfielder. The commenters, lead by Gavin (who I think still reads but doesn't comment as much, as the Pirates have worn his spirit down) argued that I was wrong; that Bay sucked. As the more advanced stats became available, it seemed more and more clear that the commenters were right. Metrics like UZR, PMR, and Plus/Minus aren't perfect, but when they all agree that a player is a bad fielder it's a pretty good indication that he's a bad fielder. Still the functional difference in defense from a good defender to a bad one is some times hard to dete...