Increasing bottom time is not why divers use pony bottles. They are not used for dive planning. There are scenarios where having redundancy with a pony bottle is superior to manifolded doubles, such as 3-tank boat trips. Swapping out the primary cylinder in between dives is more practical that having two sets of doubles.
Let's not pound a square peg into a round hole.
You have to look at the forest, not just the trees of your choice if you are going to have a meaningful conversation.
While I teach min gas in open water courses and detailed dive planning, given the poor viz, sometimes currents, and realities of not having a dive buddy in view at all times, redundant air sources have their place.
I don't think the data supports this. Bret Gilliam presented at DEMA a long time ago that the number of recreational divers far eclipse the number of technical divers, and they also keep diving for a lot longer.
When I started diving, I never thought I'd get into technical diving, nor rebreathers. But I'm in the minority here.
You need to accept the reality that the vast majority of divers are not interested in team based diving. They want to look at pretty fish (one of my favorite activities). And the largest agencies focus on meeting this market demand. Most just wanted to be herded like cats while they follow a dive guide. That's reality.
Calculdating min gas is irrelevant if catastrophic gas loss occurs. If a first stage fails shut like happened to Graham: My Own Out of GAS Experience, you better be right next to your buddy (unlikely) or have a pony (more reliable than any buddy). Burst disks failing (which I've seen), hoses bursting, etc. do happen. Just rarely.
Could you please add my entire quotes? Some of these replies are cherry picked and were answered in my view if you add the entire comment.
E.g.
Your statements:
"You have to look at the forest, not just the trees of your choice if you are going to have a meaningful conversation."
"I don't think the data supports this. Bret Gilliam presented at DEMA a long time ago that the number of recreational divers far eclipse the number of technical divers, and they also keep diving for a lot longer."
My entire quotes:
"Clarification: My argument is NOT about ascent profiles, catastrophic failures, buddy separation, or any other hypothetical problem someone on this thread comes up with to somehow detract from my original recommendation/point. However, please debate those all if you choose to; I, on the other hand, will stay out of those conversations. I am giving my example of why I do not use a pony bottle, nor do I recommend them."
The thought process for this type of instruction is to lead your divers by beginning with the end goal in mind. In my agency, most come to achieve their dreams of becoming cave and technical divers. Starting them out on the right dive planning foot (Fin) is crucial. Moreover, learning how to calculate reserve gases is a building block method that allows them to calculate further their Surface Consumption Rate, Rule of Halves, Thirds, and All-Gas Available Dive scenarios. In addition, one can now have the ability to calculate gas consumption in the team with different size tanks. With that said, this starts in what we call fundamentals and is a recreational skill. One could choose to never go into technical diving, and they would still learn how to do this in the Recreational 1/2 and, level 3 courses in my agency.
Thank you