I thought mathman nailed it early in the thread - the variances caused by group size (shooter/rifle error) and that allowed by MPBR are *additive* and so the usual MPBR story of being safely within the edge of the vitals at max MPBR range doesn't apply - at that edge range you have a 50% chance of being completely out of the vitals because of the added variance caused by group size. Using turrets to recenter the potential group at the center of the vitals would avoid this. It doesn't matter that you aren't shooting the "rest" of that group - the group size is indicative of the probabilities.
This is why I do not use a vital zone the size of the actual vitals instead I use say a 2" vital zone radius leaving a little room for shooter error. With a 2" vital zone (4" total) It gets you out to around 300 yards before you have to start spinning turrets.
I run MPBR on my rifles with turrets I mostly hunt open county glassing large clear cuts so it works for me. If I go into the timber or brush I turn it down to a 100 yard zero, but mostly I keep it on MPBR and if I see something farther I spin the turrets from there.
