Electric start systems are like automotive systems in the sense that they have a provision for rectifying the current to DC for battery charging. They still have no field winding that can be shut off and on to regulate voltage. Thats what the resistor does.
If you look at the parallel circuits and add up the resistances while dividing by the number of parallel circuits you can get the equivalent resistance of the total circuit. Knowing what the wattage of the lighting coil is and subtracting the various wattages of the bulbs, you can find the wattage the resistor needs to dissipate. If we set a target voltage of 14.7 AC for the circuit we can find the range of resistances of the "regulator" that will keep the target voltage and dissapate the right number of watts.
Like PWUstrack wrote, it is a matter of keeping those bulbs in working order, or else one has a cascade of failure.
Elan74

dear steve,
Have you seen this?
http://www.vintagesnow.com/Arctic_Cat_files/1975.pdf

As far as the other, if you want to see why, all you have to know is that the original voltage regulators used in pull start engines of the late sixties and early seventies WERE resisters. I was merely writing about replacing the expensive $20 regulator with a $3.00 part. Obviously this is for something like a rider or rat ride, not a restoration.
As for the Pantera, perhaps you could take a photograph of the trouble area and post it?
Elan74