David-
I have one car of each voltage. The main advantage of going to 12 volts used to be that you could put brighter bulbs in (especially in the headlights) and they draw half the amps at 12 volts than the same bulb brightness on 6 volts. But now, with these bright LED headlights that hardly draw much current, the 6 volt system can handle it.
Another reason is that 12 volt batteries that will fit, if you are out on the road and need to replace the battery, are much more readily available than 6 volt. But, in 50 years of touring in my Model A, I have never had to replace a battery out on the road.
A GPS or a cell phone will charge on 6 volts. Yes, they will.
Having a 12 volt CB radio or other radio is a reason.
All connections must be very clean and tight on 6 volts to keep things working well, because you don't have any extra volts to work with. 12 volt items draw less current (less voltage loss from less than perfect connection). This is especially true with the starter (lots of amps draw).
Some people will argue for 12 volts so that it is easier to get a jump off of a modern 12 volt car, or from a jumper battery pack. This is true, but seriously, if you have a dead battery, you can probably push start your A in 100 feet or less. There will be enough juice to get a spark. You don't need to jump it.
Hope this helps. I think you are right, no need to convert in many cases.
