Yeah, rear caster is a very interesting topic. Generally speaking I would think they would want the car to be a neutral in bump steer as it can be, so it would seem odd that it might have caster for that reason to me. The back has shims for the inner tie rod mounts to control that, so I would have thought they would aim for as close to zero as they can get at least. The service manual actually has the targeted bump steer toe patterns and the rear is about as straight as you can get it:
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The other thing that came to mind for me was I wonder if they put in just a touch of caster on the back to better control the toe link loading or something. Positive caster will naturally provide some self centering force to the tires (the old shopping cart wheel example), so having just a touch of positive caster would try to keep the wheels pointing straight instead of pure zero which might let them wobble a bit left to right. They wouldn't actually really wobble I don't think, but it might sit there and apply an oscillating force on the toe link that would work it back and forth and could maybe prematurely wear it out? Having just a little auto center might keep the link loaded more constantly. This is all just theory in my head from what I know about suspensions though.
Really, when I think about it, camber is also going to be slightly affected by chassis rake. I have a little jig I use that just attaches to the wheels and you put a level on it to read camber, but whenever I do that I make sure I first set the jig perfectly vertical on the wheel before measuring the camber. However, the adjusters are actually going to adjust that relative to the line between the ball joints defined by the caster (at least theoretically if you adjust both cam bolts the exact same at the same time). I need to get out my old race car vehicle dynamics book and read over it again though. I'd say maybe all of this doesn't matter at all since the values are likely so small, but then again we're trying to set stuff to fractions of an angle on alignment and 64ths of an inch when we do toe. I use the comp coupe shims on my car as well instead of the cam bolts, so I can adjust stuff "exactly" at the same time. When I was setting up the caster on mine for the track this past time I would basically mirror the shims to get the caster right and keep the camber the same. So for example if I had a D shim and needed more caster I would put an E shim in the front and a C shim in the back on the same arm.