My 2¢,
The polarity reversing switch was used to increase the life of the points, so the build up was not just on one side of the points. That's my understanding, although I could be wrong about this! I know the points on Model T coils have a longer service interval when the car is run on MAG (AC), instead of BATT (DC).
I don't think most people use them anymore. If you've got one, it would be cool to wire it up.
I could be all wrong about this!
If I am, someone will point (hahaha, get it?) it out.
