There was an occasion when I needed to shutdown my gas engine in flight in order to land the plane after the throttle linkage broke. The remote kill switch saved me from flying around forever at very low speed and not knowing when the gas would run out.
Also, fully closing the throttle doesn't always shut off my MVVS 26cc gas engine after a hot flight on a Summer day which makes the transmitter's throttle cut feature ineffective.

RCATS makes a fiber optic electronic kill switch that sells for $49.95 but I use their wired RC100 - electronic kill switch that costs only $24.95 ( http://rcatsystems.com/electronics/rc100.php ). The connector plugs into a vacant receiver channel and the other end is wired in series with the ignition switch.
It works perfectly in my MVVS 26cc Ultra RC Giles and it has a green LED that indicates when it is turned on.
The MVVS doesn't create any electromagnet interference issues even when using a plain ole PPM receiver.
The photo shows the tiny light weight kill switch next to a penny in my plane.