Dr. Manhattan notes something I’d thought about myself: that Tom Tippett’s analysis of balls in play against pitchers (which I noted here) — which concluded that at least some pitchers do have an effect on balls in play, in a revision to Voros McCracken’s groundbreaking theory — is significant because many of the recent advances in fielding statistics have been premised upon the idea that the fielders alone control a team’s overall rate of hits on balls in play.
Of course, still absent (I think) from a lot of the analyses of defensive stats is the other wild card: park effects. Until we make sense of the components of park effects, we can’t really unravel the balance between pitchers and fielders on balls in play.