Never flew a turbine helo that had a ignition switch. Never has trouble ever starting a 212 or 412 (non fadec PT-6), no tendency to over temp, but that was all sea level operations.
Fixed wing manuals I have powered by PT-6 all have separate starter and ignition switches, one single engine even has a caption warning if the ignition is on when the engine is running with the advice if not flying in rain then turn it off. Outwest, the start on the Thrush is exactly as you describe for the Air Tractor.
King Air system.
The engine ignition system is a high-energy, capacitance type consisting of a dual-circuit igniter box and two igniter plugs in the combustion chamber. The ignition system is divided into starting ignition and autoignition.
STARTING IGNITION
A three-position lever lock switch for each engine controls this system. The switch is located on the left switch panel. It has three marked positions: ON–OFF–STARTER ONLY. The ON position (UP) is lever locked and it provides for engine cranking and ignition operation. The STARTER ONLY position is a momentary (spring loaded to center hold down) position and it only provides for engine motoring. In this position, the igniters do not function.
AUTOIGNITION
The autoignition system is controlled by a two-position switch for each engine marked ARM and OFF. Turning on an AUTO IGNITION switch arms the igniter circuit to an engine torque switch that is normally open when the engine is developing more than 400 foot-pounds of torque. The system must be armed prior to takeoff and for all phases of flight, and it should be turned off only after landing. If engine torque drops to 400 foot-pounds or less when the autoignition is armed, the ignition system will energize to prevent engine flameout if the loss of power was caused by a momentary fuel or air interruption.