Uh huh! Nice analysis, Hello Sailor! At least as far as it goes...
Now, let's get real world. Quite apart from cost (2-3 times at least) and size (where the hell do I put these tall batteries) and maintenance (12 cable connections to inspect and maintain instead of 2), what happens when you lose a cell during a passage?
You're two days out of LA headed for Hawaii, and one of the cells on your ONLY 12V house battery develops a short. This might be because of bouncing around shook something loose inside, sulphate buildup in the bottom, plate corrosion, etc., etc. In any case, instead of 12.6VDC you now have a 10.4VDC house battery bank. Not enough to run your SSB, or other appliance which needs at least 11-12VDC to operate.
You're many days from shore. You're many more days from obtaining a replacement for the bad cell. Oh, bother!
With a paralleled house battery bank what do you do? You just flip the battery switch from "ALL" to "1" or "2" to isolate the bad battery bank, and carry on as before.
Do you think the sailor with the paralleled battery bank gives a damn that he/she may be losing a tiny percentage of efficiency by paralleling the house battery banks???
S/V Born Free