Well, first both you and Andrew 1DA are correct in that most times a single fuse is sufficient. The car case is the classic where a chassis bonding fail can cause huge currents to flow from the mobile antenna chassis connection through the screen to the mobile radio and from there through the mobile radio chassis and earth print traces back to the battery. Hence the negative fuse lead in addition to the positive.
My case was a derivative of that and somewhat contrived in that connectors had been removed as I forgot a battery. If you only ever run one piece of equipment off a battery it isn’t going to happen. For me, a bag of 10 IDC automotive blade fuse hoders and 10 5A blade fuses was pennies and because the setup described in the linked thread may arise again (needing to power transverter and IF radio off one battery) it seems like a pragmatic fix to me.