The defensive metrics only include fielding(I believe). They don't take into account scooping balls out of the dirt or saving errors on errant throws. With an infield that had all average or better fielders it wouldn't be much of an issue. However, we have Weeks at 2B and Hall/Lamb at 3B(I know Hall is solid for the most part). Hall and Weeks are prone to making bad throws that an average or better 1B could catch and make our overall infield defense better. As it is, Fielder just compounds the problem. How much is tough to sort out. That isn't to say he has negative value, but he has to hit very well to make up for his defense and why many people feel that an average hitter who could play good defense would be almost as good as having Prince at 1B.(and likely come much cheaper) We have Prince for right now and I am ok with that.