The best defensive metrics use play-by-play data that shows were the ball was actually it and how hard. PMR, Dewan's +/- system (used in the fielding bible) and UZR come to mind. James' Win Shares just uses put outs, assists, etc.. for his defensive win share calculations. That kind of approach is very faulty because it can't even measure how many opportunities a fielder had. How James calculates the value of defensive wins shares relative to pitching win shares, is very arbitrary. He just assumes a constant ratio IIRC. James has done so much outstanding work in the field of sabermetrics but Win Shares doesn't seem to be one of them. Here's some more information, straight from the horse's mouth, when James introduced Win Shares at the 2001 SABR convention in Milwaukee: LINK