You can't practice anything by creating a real emergency ( well, I guess you can...).
I was trained back when some of the training is now considered hazing, and if not done properly, and the student trained properly prior, I would agree. The result was dealing with a real OOA and other casualties, and although it was a controlled situation, your initial body reaction is panic, and the training was to deal with that so you could act properly when it happened.
OOA is a real emergency now, when I started it was just one way to end a dive, no drama. Since it happened a lot, the training focused on getting you to the surface alive and well.
I encountered the same type of training later on in the Navy for fires and flooding, shut in a compartment with a fire, flooding, or both and have to stop the emergency. Now I'm pretty sure the instructors would not let me die, but at sea there were no instructors to end the problem. I liked the practice.
Then even with the practice you still don't know how you'd react in a real situation and hope you never find out.
Each situation is different so ultimately you can't really know until the s**t hits the fan, but if you train and practice you have the tools to extricate yourself from an emergency if you use them.
In diving, and other endeavors, I have found, in an emergency, that I'm good as long as I work the problem. However I have seen panic ahead when I'm running out of possible solutions, luckily I resolved the situations before I panicked, I don't believe anyone is immune.
Yes, 3 cuft. has to be better than 0.
At the right moment it could save your life.
Sure, a Spare Air doesn't hold anywhere near enough gas to be considered a reserve.
Depends on what one considers a reserve, with the proper prejudice, one could say that about any size. 3 cuft isn't a big reserve, and perhaps less than what becomes necessary, but it is a reserve.
Bob