If you lose the throttle servo you can't control engine rpm, correct? Just where does one cut the engine and at what speed will that happen when the decision to land the plane takes place? Personally I have not been able to find a benefit to additional kill methods beyond idle cut off and the external ignition switch. Unless one is using a tank much too large for the application or engine flight times would not generally exceed 30 minutes at full throttle. At reduced rpm levels it's possible to put the mains on the ground and hit the prop with a dash of down elevator. Odds are high that should an engine have the engine cut in flight from a choke or optical switch the prop would get broken in the ensuing rush to get the plane on the ground, along with some bent landing gear.