A little surprised we're 50+ posts in and no one has pointed out the obvious -- why Pop started Bonner.
The rationale is, imo, obvious. With no David Lee, GSW has no bigs who can defend outside the rim. Not outside the paint... outside the rim. Bogut's extremely effective under the rim, but doesn't have the quickness to be effective anywhere else. It's why we've seen Duncan get so many shots from the top of the key; GSW would rather keep Bogut inside to collect rebounds and clog the paint to prevent Parker drives. And it's a good strategy too.
So, how do the Spurs counter that? By sending in "another" offensive floor-stretching big. Duncan by himself is a reasonable floor-stretcher for a big, but you can easily use help defense to mitigate that. But put Duncan and Bonner out there together, and the Warriors, THEORETICALLY, are essentially forced to make their defense spread the floor as well. Someone has to cover Bonner out there... that, combined with Duncan being at the top of the key, leads to HUGE space for Parker to drive.
The question then becomes, what affects Bonner's +/-? Given that he doesn't shoot much, this should be obvious.... Bonner's +/- is almost entirely dependent on if Parker and the guards are successfully driving the lane, and if Duncan et al are hitting their jumpers. In Game 2 --- they did not. It's not Bonner's fault. Just like it's not to his credit if it's high. He's practically a decoy.