It just seems that offensive formulas are "easier?" to quantify, and are perhaps more readily accepted. Offensive metrics are in a much more controlled environment. You know the pitcher's prior success and you know the batter's. That dreaded BABIP comes in a ruins everything, but compared to defense, it's a piece of cake. We don't even HAVE most of the information you would need to properly gauge defensive abilities. We finally have spray charts available but what was the velocity of the ball hit? What was it's trajectory? Where was the defender positioned? Where did the defender end up? Too many variables and almost no data. When statheads have tried to make up metrics using the data available, they have had limited success. They certainly aren't good enough to make the proclaimation, "Player A is better than Player B!"