This subject has been brought up many times not only on this list but others as well.
Probably in more cases than not is the fact that it is psychological. Many people think that when they hear a 6 volt car in the process of starting, that it is turning over too slow and probably isn't going to start. The fact is, it isn't going to sound like a 4 cylinder 12 volt rice burner or any other 12 volt car in the process of starting. Think about it; with a couple of exceptions including the 53 Crown Imperial, all American cars were 6 volt up until 1956! There was no alternative and everyone got along just fine. And nowadays, unless you have a very tired motor, poor grounding, or other wiring problem the 6 volt system will start your car just fine, and run an air conditioner as well. The negative of course, is if you want to install 12 volt accessories like sound systems, etc. I have owned many 6 volt cars over many years, and have never had to convert it to 12 volt. Again, I believe it is simply a mind set over the way it sounds when starting.
Bill B.