I'm not quite an advocate of Spare Air, however I have owned one and used it, although not in an emergency as I try to avoid them. It would have been handy back before I had an SPG, when emergency ascents were more prevelent.
For practice CESAs it makes it easier than without, which is the reason I tried one, but I found that the regulator was not as robust as the usual second stage, and the particulate, including sand, in the where I dive, compromised it's relialability.
Somehow the argument seems to be that a 3 cuft cylinder is not a 30 cuft cylinder and therefore not usefull. Overlooked is that 3 cuft is much better than 0 cuft, especially if the diver is not going to use anything larger. Also overlooked, is that one diver's definition of the proper kit is different than another's, and each has to decide what is right for their dive plan.
My personal argument was, since I was not going to carry anything very large, the Spare Air was unobtrusive. I know how much 3 cuft is, and I was only looking for another breath at the right time. The alternative, when the Spare Air proved unreliable for me, was back to diving without redundancy on rec dives, buddy and solo.
Decades later I decided to carry a 19 cuft pony on deeper dives as I'm not as young and fit as I once was, the jury is out on smarter. And some will tell me, even now, that my pony is too small for my dives.
Yes, agree with all this. I've often mentioned that I practice CESA starting with less than full lungs. I agree that while not a real emergency when your mind may be flipped around, it's better than nothing. I would figure same idea with Spare Air, as you have tried using it just to see. Or even my foldable snorkel, just to see how it feels. One can't practice anything by creating a real emergency (well, I guess you could....). That relates to rescue skills as well. You practice them and may never use them, but better than not practicing them. Then you wonder if you could still do them in a real situation. Hopefully you'll never know. Yeah, 3 cuft has to be better than 0.
I don't mind the discussions about Spare Air as it gives information on gas planning for decision making. I do think that after one weighs the information and decides on the SA, it is an informed decision and it's time to move on to the uninformed. We're all going to die, and I don't think it's going to be from SA, or some of the other gear choices that the majority of of the ScubaBoard members jump on.
Bob