"The coil builds up close to the same amount of voltage whether the spark plug is attached or not. The coil has insulation in it to prevent internal sparking. End of story."
NO. lets say that if the coil had about 5kV(i know how the spark is formed, there are no 5kV all the time
again i'm not sure if its 5kV ) when working normally(connected to the plug).
if you remove the wire, the coil will have about 20-25kV(not sure of this, but still enough to jump INSIDE THE COIL, making a route for future jumps inside the coil) this will result to instant junk coil, or couching acceleration (when you quickly turn the throttle, the mixture will go to lean for a very little time. at that time(mixture lean) the spark will have to be stronger to jump at the tip of the plug, so it might jump inside the coil(if the insulation is damaged, here you are right, but the insulation can be damaged by running the engine without wire plugged to one of the plugs.) and therefore cause poor acceleration or stalling.
had a little discussion with a car mechanic 10 minutes ago
the kilovolts are so high, because the spark normally can jump when only (as said 5kV for reference, not sure of the right value) 5kV is achieved. but if the distance is more than the little space at the plugs tip, then kV:s will go high and MIGHT(usually will) jump inside the coil.
could you now believe me