It's twenty nine years since I flew the Wessex 5 but I do remember doing "PERRRFs", which stood for Port Engine Rotors Running Re-Fuel, which was one occasion when the procedure described above was used.
As I recall, this was necessary because of the mechanical complications caused by fitting an airframe and main transmission originally designed for one engine, with two engines. A coupling gearbox joined the two drives together. The best reason I can think of for fitting free turbines - far less complicated!