Nothing like giving yourself an engine failure..not sure I agree with this instructors philosophy.
I've done it dozens of times, no big deal.
Some aircraft dont even have engines, yet fly very well.
I understand that a cessna will glide, however, why put the aircraft and student at risk when you can simulate the engine failure?
It's an easily managed risk that builds confidence in the airplane and one's flying abilities.
You are missing the word "unnecessary" there.