I taught lots of different courses for both for Naui and Padi.
I am an expert at what I did.
I taught surf entries off of Southern California beaches, AND I taught boat diving off of Oahu. I did not ice dive, mixed gas dive, quarry dive for cars, etc. I hate the taste of icky fresh water!

My point to all this is that most of my students tended to dive in less then 40 feet of water; there was more light, and much more sea life for them to enjoy in the ocean.
I taught basic, advanced, and specialty classes mostly; then I taught Instructors, and taught them to teach those classes too.
When I had students in classes, I absolutely had my spare air;
I had an Air ll as a secondary off of my tank, also.
My goal was to be streamlined and efficient;
it's the example I set for my babies.

Spare air was and is a useful piece of equipment for what I did.
Practical, light, and streamlined, I have used them for everything: as a teaching tool, back up for free dives, even for loosening the anchor at the end of the day.
And of course it takes practice and getting used to;
just like mask, snorkel and fins...
If you have the desire and training to dive deeper, by all means, use whatever works for you. Just don't be dismissive of equipment you haven't used, or that doesn't fit your type of diving.
Best Fishes,