Quick Story here: Last year another instructor and I were teaching an O/W class. I was in a Back inflate, the other instructor was in a BP/W, and the students were in Jacket style BCDs. We went over the differences in everybody's gear. Nobody was confused. Just by comparing gear, information was passed. From a sales perspective, I think that focus on the gear helps sell it.
I'm lucky in that the shop I teach for sells great equipment, and just about everything I dive is available through the shop.
That said, when I teach in the pool, I use a jacket style that the shop sells. This was simply because I have a set of gear that I leave at the pool, and this made the most sense. When I'm going to be demonstrating every skill, I'd prefer that the students see me doing it in the same gear they are using. In other words, it's the diver, not the gear, though I do explain that my pool gear is not ideal for Northeast wreck diving.
As for the snorkel, my agency requires me to have one, and I do, in my pocket. Some people prefer to wear one, some don't. It's another opportunity to explain different preferences that people have. It's also an opportunity to explain just how strongly held opinions on the whole issue are!
Bottom line, though is that shops have to set guidelines about the issues we are discussing here. If you're not comfortable with the way the run things, you shouldn't DM for them. It shouldn't be too hard to find a shop that you are comfortable with.
Please don't stop being an active DM. DM's really make a difference with students and instructors.