I've never thought that Bavetta blows calls. I think Bavetta is constantly looking for opportunities to keep games close and that he chooses what to see and what not to see based on the game situation.
The most vivid example of my mind came last season in a game between the Spurs and Clippers in SA. The Clippers scored with less than 20 seconds left to take a 2 point lead and the Spurs called timeout. During the timeout, I turned to the guy sitting behind me and said that I figured Bavetta would find some way to put Duncan on the line with enough time left that the Clippers would have one last possession to try to win the game if Duncan made both FT. Sure enough, after the timeout, Bavetta called a foul on Brand on a play where he was defending Duncan near half-court. It was a sketchy call all the way, but with Bavetta in the building it was perfectly predictable. He got a game that was either going to end with some drama in the last 30 seconds or go to overtime.
I think sometimes Bavetta believes that his obligation is to ensure that possibility as often as he can, even if it means that he's inconsistent over the course of a game.
As most know, it's not a coincidence that Bavetta seems to end up working game 3's in series where a heavy favorite has decisively won Games 1 and 2 at home -- in the Spurs' case, games like Game 3 of the 1999 Finals; Game 3 of the 2001 First Round series against Minnesota; Game 3 of the 2005 First Round series against Denver (a game with some ridiculous number of called fouls).