I am attempting to install a new engine kill switch into a 1990 Malibu Sunsetter with a Mercury MIE 5.7 LITRE SKI inboard engine. The Kill Switch is a Sierra Emergency Cut-Off Switch (MP40970-1). The Switch's installation instruction specifically mention not to interrupt the purple wire connection to the instrument or alternator circuit. After inspecting the wiring diagram and actual wiring for this engine I find that if I interrupt the purple wire from the ignition switch and after the splice for the instrument panel that I will interrupt both the ignition coil and alternator.

Is there a reason that it is not okay to wire the Cut-Off switch to interrupt the purple wire attached to the alternator? It seems to me that in an emergency situation that it would be okay to interrupt this wire connected to the alternator. Is there a limitation to the Cut-Of switch that somehow may effect normal operation?