Keygrip.
I wouldn't say, in particlar, that the exercise is to show competence in flying at Vmca - it's more to show why you SHOULDN'T fly at, or even near, Vmca
Risking the neck of students (think of the legal ramifications if someone gets killed) to show them why you
shouldn't fly at or near Vmca is similar to feathering both engines to show you the dangers if they don't unfeather.
And I know of one idiot who actually did this while he was conducting an instrument rating test on a Duchess. He made the student try to unfeather one engine with minimum loss of height. I recall the adage about old pilots and bold pilots but not too many old and bold pilots. Unfortunately the instructor on the feathered engine instrument rating test himself died a few years later sticking his neck out that little bit too far.