If they can't handle 6m/20ft and problems with a wing, then it's probably better that they don't dive in the nasty real world where you can't blame someone.
If the instructor does this, he should probably warn in advance that he plans to do such things in the briefing.
Its a big call for an instructor to make a psychological assessment ( an area that they are not trained in ) that this person wont panic if i do this - ive seen very experienced divers stress out over things that they "should" be able to manage. Id say if your going to give them ( a beginner) a stress test or equipment malfunction test then It needs a build up and that often involves warning people in advance, then one can have some feel for their ability to deal with the next level of stress test.
Having said that an instructor with many years of training is likely to be able too gauge that better then an freshly minted instructor