Only that it is cheap and easy for the manufacturer and when boats came with only two batteries and a 1/2/BOTH/OFF it allowed the user to select where the charge current went. Bottom line it is cheap and easy and builders still do that to this day.
...The only thing that hurts by going bigger is the wallet..
Thanks again, Maine Sail! OK, so when I rewire, I can skip the route to the solenoid, and run my 4 AWG output from the alternator directly to the POS busbar or battery--where the MRBF fuse will be. I plan on a 125 amp MRBF for the 4 AWG wire.
Some older Balmar's had a case ground option but most still shipped with a ground stud.. If there is a ground stud on the back of the case make good use of it....
I did some more research--I think the alternator is a 9075, which did indeed come with the case ground option. I will check if there is a ground stud though. If there is, I'll run 4 AWG cable to the NEG bus bar.