Injuring trainees is generally frowned upon these days. Endangering new divers by intentionally harassing them is both unnecessary and potentially dangerous.
Injuring trainees has always been frowned upon.
Divers are not endangered when instructors trained to manage such harassment properly execute it.
Diving is "potentially dangerous" and what helps eliminate some potential dangers is thorough training.
I'm sorry, but as an instructor trained to manage the worst case scenario of a diver becoming out of control in a cave environment due to panic while being expected to teach a failures base class, I cannot rationalize how an open water instructor is not helping a student to be safer by creating such failures as:
1) Air Gunning - Using an air gun or regulator to mimic a first stage or tank O-ring failure in class will reduce the stress of a diver who actually may experience one in or under the water.
2) Leaking or flooded face mask - once a student has been trained to clear a mask and is proficient, a random skill drill of hovering over the students and pulling masks away from faces to leak or flood them will show the instructor that the student is comfortable with surprise mask problems. This is best done as an expected exercise. Once the instructor is sure the students can handle unexpected leaks and floods, unexpected loss of face masks is the next skill building exercise. This is the level that a student should be at for OW diving comfort and safety. To reinforce student comfort, after this has been executed to the instructor's satisfaction, the exercise can be thrown into the mix of other exercises once those exercises have been demonstrated to be comfortable and proficient. Example: Diver who is comfortable with unexpected loss of mask and who is comfortable with removing and replacing scuba can be challenged with a lost mask while replacing scuba.
3) Lost power inflator - unplug the student's power inflator during ascent and buddy breathing ascent skills to make the student proficient at going to the oral inflator. This comes after the student is used to unplugging and plugging in his own inflator proficiently.
There are other harassment skills, but these are three examples.
What I ask myself as an instructor is, "Is this student ready to handle the problems associated with equipment malfunction during open water diving and will this student be able to comfortably manage them easily and proficiently? Has the escape instinct been trained out and replaced with an automatic behavioral response that a diver should have?"
If the student does not automatically resort to comfortable and efficient underwater problem solving, diving is "potentially dangerous" for that student - especially if the student still has an instinct to escape.