I certainly do point out that at some point the diver will have to release the pinch to get the mask on anyway, but by starting with the pinch (and then having them release it out of boredom more often than not), I am hopefully ingraining a first response to water in the nose that stops/prevents panic.
Here's my setup for the skill (off topic but on topic too, maybe).
From the very first breaths on scuba, the students are standing with no mask on, and put a regulator in and pinch their nose. They drop their face in the water and breathe for 10 minutes (or really as long as it takes for everyone to stop the pinch from sheer boredom, or at least manage it for a little while, which can be much shorter or longer). I encourage to try releasing the pinch, and then repinching if needed.
But I have had guys who pinched their nose, even with the mask on, for most of day one all throughout the open water dives as well. If they had not learned the nose pinch self-rescue technique, they probably would have had to quit before they ever got certified, because they only other solution is to ask a panicking person to relax which is like shouting at a mountain for being tall.
Since I also had the inability to mouth-only breathe at the beginning, the only way I got to even be able to snorkel was through living on the beach and going into the ocean every day for six weeks (and the only reason I could do that is because I was living literally on the beach in Hawaii, which is an opportunity very few people have, both the time and the place.)
And every day getting into the coean, spending ten seconds at a time with a mask on, sticking my face in the water til I stood up literally gasping for air. It's one thing to know I can breathe only through the mouth. It's another thing entirely to convince my body of that, as I found out. I was thinking about just using goggles and living with blackeyes to see the fish.
The reason I became an instructor (and the reason the guy mentioned above became in instructor as it turns out) was because most instructors seem to have no idea how hard mouth only breathing (and therefore mask clearing, and mask R&R etc.) is to certain people. It's not fear, or discomfort, it is the simple fact that some people have used their noses their whole lives, and in the course of a short scuba course we cannot retrain a lifetime of habits into something new. We can however teach people a coping mechanism that solves the immediate problem until enough diving allows no pinch no mask breathing.