If demonstrated at safe height the excercise provides a useful illustration to the student that there is a speed below which he cannot control the airrcaft in the event of assymetric flight
The problem is specifying the safe height. A light twin "departing" controlled flight may need anything from 1500 ft to 10,000 ft to recover from a spin and by then you are in test pilot territory and even he would have a parachute! Surely a well designed computer generated image of a Vmc demo would be a great aid to flight safety especially if it showed what happens beyond Vmc if the aircraft gets away with you.
Meanwhile it would seem prudent for ME instructors to stay away from Vmc especially as normal asymmetric flight requires a fair amount of corrective rudder making it obvious to a student that running out of rudder is not a good thing.