Always looks a little odd when viewed as an isolated statement of fact. Just try thinking WHY such a condition would occur - given the natural redundancy levels of the 47 electrics.
IF such a condition occurred, it would be because of a pretty catastrophic electrical problem and what you then want is separated systems, to prevent one system's problems taking out one or more of the others. Sticking to the 'book' selections will give you a good base from which the FE can start to diagnose and, perhaps, re-instate working systems. It's always worth taking on board the TWO functions of the SSB - connecting up in parallel when all is going well, BUT, when it turns to worms, giving you four separate sytems, any one of which will get you home.