At the moment of discharge, the high electric fields between the plug electrodes induce electron collisions that begat more electrons (the so-called avalanche effect) that, within a few tens of nanoseconds, result in a violent hot gas. If the ignition system is undamped, and free to supply current to the electrodes, the discharge will continue well past the time needed for ignition and flame-front propagation. This condition results in several milliseconds of optical (the so-called "fat" spark) and high-frequency radiation. While it's satisfying to see in a dark garage, the "fat" spark is inefficient, as the flame-front has already propagated throughout the cylinder by the time the spark has extinguished. Moreover, the high-frequency radiation generated by the discharge disrupts some electonics (there is a large amount of energy radiated at 1 mHz, for example). To stabilize the discharge and limit its duration, resistive media was introduced in the 60's; in the high-tension wires, in the caps, and in the plugs themselves. This resistance, typically between 2 and 5 kilohms, has no effect at the moment of discharge. As the discharge progresses, however, it limits the current to a value below the ionizing holding current, and the discharge terminates more quickly, limiting the average radiation. Nearly all "exotic" material automotive sparkplugs (iridium, platinum, etc) contain an internal resistive plug of a few kilohms, and should not be used with either resistive wires or caps. These plugs are ideal for our bikes.
"Suppressor" plugs are a specialized type intended for weak CD systems, mostly marine, where the discharge voltage is relatively small, and a "fatter" spark is required. The discharge here is limited by an internal inductor. I don't see the application to motorcycle ignition systems.
