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In a perfect world you would use a global MFA on each camera body you own to bring it to "dead center" and assuming all lenses had the MFA feature (like new Sigma) you would adjust all your lenses to dead center. Every lens would work perfectly on every body. Actually, if it were a perfect world these products would come from the factory with much tighter tolerances and users would not have to waste time doing any of this :-)
In the real world, bodies and lenses each have a focus accuracy margin which in some combinations produce sub-optimal outcomes. Most high end bodies now offer MFA (either global or per lens) allowing users to dial in the optimal settings (a time consuming and tedious process). To your point, the new Sigma MFA feature does offer value for bodies without MFA. I suspect, however, that the vast majority of folks with the means to buy a new 120-300mm will also be using it on a body with MFA. I think those folks will choose to MFA their bodies rather than their lenses so they can simply move a lens from body to body. Folks choosing to MFA their lenses will have to connect to the USB dock each time they want to switch bodies.
Listen, I am not trying to dissuade anyone from MFAing their new Sigma to their heart's content. I just see little value to this feature for folks who already own MFA bodies (me, for one). Furthermore, I am anticipating (perhaps prematurely) that the MFA process for a dismounted, USB dongled lens is going to be even more tedious than the already tedious process of in camera MFA adjustment with the lens always mounted to the body. I will retract this concern immediately if the new Sigma software allows you to mount the lens, run their application, dismount the lens, attach the USB and download the magic numbers (in fact I will cheer for them). If, however, you need to move the lens back and forth between body and dongle to make, test and then download refined adjustments, I think this "feature" will see very little use. I am very anxious to hear from anyone with the new dongle and software as to how hard or easy the process is.