RAC1812,
On a 12 volt system, the ignition was designed to run around 8 or 9 volts. This is obviously below the actually running voltage of the system.
To reduce the voltage to the ignition, several methods were used...a coil with a internal resistance built in, a resistance wire that goes from key to coil, or the traditional white ceramic ballast resistor you are probably describing as seen on the MoPar rigs around the '70's, and bazillions of Chevrolets.
Without the reduced voltage, the points will burn and wear at a accelerated rate. I actually don't know if the coil proper will generate more heat and fail faster, but I suspect so.
So....if your system is 12 volt, then some device needs to be on board to drop the voltage to the ignition system.
Your coil statement is interesting. I have only seen coils marked specifically for 6 volt or 12 volt. If you see the impedance specs for the different coils, they are quite close regardless of voltage, so I really don't know the answer to that question.
Stuart
