how many dives have you carried a pony and how many times have you needed it?
When looking at dive accidents and incidents, typically it is never a single thing that goes wrong. I even heard something very similar recently around aviation safety on a podcast I was listening to. A "3 strikes rule," which is a good quick and simple way of explaining it. It's rarely ever a single thing that goes wrong, because a lot of scuba equipment and procedures are designed around redundancy. Instead, it's a series of things going wrong, perhaps 1-2 under control of the diver, and another 1-2 out of the diver's control.
For example, first the diver was tired, then the diver got cramps, then the diver had buoyancy issues, then the diver passed up multiple opportunities and suggestions to turn the dive, and then the diver finally had another buoyancy issue, shot to the surface and died. Or, divers have a minor equipment issue before dive but ignore it, decide to cave dive without training or proper equipment, get lost in cave, and drown.
Instead of 3 strikes, I like to think of it as "snake eyes," where if your dice all roll 1's you're dead. To improve your odds, you can add dice by adding redundancy, or increase sides by improving quality of that redundancy. The whole "never dive alone" mantra repeated by every dive agency over-and-over, is an example of that redundancy, and of course a better buddy would be an example of more reliable redundancy.
When it comes to life and death, and in SCUBA, we're never that far from it, you would hope you're never in a situation where you really need your redundancy. However, if you ever do need it, it may save you from death, severe injury, or a traumatic experience. Regardless of whether I'll ever "need" the pony bottle, it certainly is a stress reliever in that I don't have to worry about a large class of potential scuba issues.
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Now to actually answer your question directly:
I have carried redundant air (sometimes a pony) on every dive since about July 2021, which is about 100 dives (not sure, I don't log/count). Around June 2021, I had an incident
described here. Thankfully, I didn't "need" redundant air, because I had none with me, and the redundancy of the surface being about 30ft away was good enough. However, if that had happened on one of my few 60-120ft dives (I had several at the time), the risks would have been much greater.
Many divers treat their dive-buddy as a swimming pony bottle, and there is certainly no shortage of those stories.
I am talking about NEEDING it.... lol
I also know precisely what you're doing, and you do too, setting an unrealistically high bar. If divers "needed" pony-bottles more often, we'd probably have a LOT more dead divers. If someone needs redundant air, and doesn't have it, they end up dead or injured and wouldn't be able to answer.
As seen by other responses, people have actually needed redundant air / pony-bottle, or at least found it extremely useful.
