Multi Spark - MSD being the best known - is multi spark because the spark duration of the system with the type of transistorized ignition configuration they used is shorter than optimum for best ignition. IOW the spark does not exist as long as it should SO...they "invented" multi spark to compensate for that short spark - essentially "hooking together" several really short (in time, not distance) sparks to create one good spark.
If you think about how ignition works, a second (or third or fourth) spark in the sense of really being separate "normal" sparks cannot do anything useful. The flame front has already ignited with the first spark, subsequent sparks can't ignite anything because there is nothing there to ignite. The Multi spark is simply several sparks done so quickly as to become one "normal" spark at the correct firing time.
It is true, of course that a higher powered ignition circuit will ignite a mixture under adverse conditions where a standard ignition might not. High compression situations come immediately to mind. So, as noted, a higher power ignition system will be useful in situations where the standard system is not up to the task. Also, as mentione, opening the plug gap can improve the flame propagation in the cylinder depending on cylinder design. As is true of so many things, it takes some experimentation to determine what works best for any engine in whatever application. If you want to seriously get into spark plugs, we have to talk about indexing!
Yes, it works...on some engines, not noticeable on others. No idea re Nortons.