When I used to instruct, years ago, I almost always did this to my primary students at least once. Note, I always had a suitable landing field in range, we never stopped the prop, and we never actually continued the exercise to landing in that condition. The point of the exercise was to see if the student remembered their training, and found the offending fuel selector in the OFF position, or if they just sat there imitating a goldfish, (mouth moving and no sound.) Most often, I got the goldfish reaction!
I do think it was a worthwhile exercise, it drove home the point to follow your training and procedures, and to FLY the airplane. Any actual practice forced landings were done with the engine idling and my hand on or near the throttle. I also never had an engine so much as sputter momentarily when the fuel was turned back on. (These were all carbureted, simple engines.)
My first ME lesson was in an old Apache, and toward the end of the lesson, the old instructor slapped the right engine mixture back to simulate a failure. All went well until we tried to re-start same, it refused to do so. After an uneventful SE landing at nearest airport, some 10 miles away, we found the mixture control cable broken off at the carb. So my first ME lesson was quite a learning experience.
