There are three basic types of 'kill switches' out there, and have different applications it seems.
 
1.  The original "magneto grounder" which is an on-off switch with one end on magneto and other end to ground, when closes/activated the magneto is grounded and spark generation ceases thereby killing engine. 
2.  The "optical kill" for electronic ignition set-ups.  Works good but I tried once using one to ground a magneto engine and it didn't work, my electronics genius cousin suggested the switch it intended for lower voltage/power applications only.
3.  The High Power "Optical Kill" which can be used for electronic ign setups that most of us use on bigger engines but also works as a magneto grounding switch if you want to run it this way.  I found one of these a few months back at TBM and it solved a problem I was having where the std 'opti-kill' wasn't cutting-it (no pun intended)

I think a secondary kill system is neccesary and desirable.