Ford started that stunt with the Model As (my dad and I blew up a cheap battery charger when we were in a hurry and hooked his AAs battery up backwards). With the really old technology batteries (where you could by the internal parts separately and rebuild your own), it kept the terminal corrosion down, which kept corroded salts from playing havoc with electrical connections at the battery. 6v systems are MUCH more vulnerable to bad connections than 12v.
