The fundamental insight that spurred the development of this metric was the realization that balls in play that turn into outs for every pitcher tend towards the same value. The primary variables are: 1) type of batted ball (gb, fb, ld) and 2) defensive efficiency. #1 varies by pitcher and #2 varies by team. Not every individual ball has the same chance of becoming an out, but over the course of thousands of balls put in play per year, this has been repeatedly shown to average out. What this means is that looking at the efficiency of the defense is a very valid metric. I understand your stance, and your opinion - that's not the problem. I'd like you to answer the question about where overthrowing cutoff men, throwing to the wrong base, Barrett calling 8,000 fastballs (on 0-2 and 1-2 counts) a game, Pierre playing about 10 feet behind second base, etc etc comes into play by numbers? Edit: Meant to type 0-2 and 1-2 :D