While you have a very valid point, IMHO if you have all brand new batteries, this doesn't really happen.
Not true, Certain AGM batts may rest at 12.8 while certain wet cells may rest at 12.6. Even brand new, fully charged. DONT MIX TYPES
I have AGM batteries in the back that rest at 13-13.1, and my front flooded battery rests at 12.8.
I can turn off the car, come back 2 days later and everything is still sitting at 13-13.1. So whatever parasitic draw caused by the front battery is negligible in my case.
The life of your batteries are being cut down