You should turn off an airplane by fully leaning out the mixture. Yes you can turn off the engine by turning off the magneto instead of leaning out the mixture. However the carb will be ready to go, fuel will stay in the manifold, and thus if the switch is bad could start accidental. This would make the error of operating the aircraft with a bad ignition switch doubly bad. If the carb and manifold are full of fuel, the engine at the right point, and the ignition switch bad, then it could conceivably start with just a gust of wind to move the prop a few millimeters.

BTW do new aircraft still use those lousy Slick and Bendix point driven mags?