I think it must be a regional thing. The people I most often dive with (PNW) carry Pony bottles, the main conversation about them is whether they should be slung like a stage bottle, or attached to the tank.I think it's a regional custom.
Similarly, I have seen posts from people talking about dive boats requiring pony bottles and everyone they knew using pony bottles, etc. They write as if everyone uses pony bottles. I have never been on a boat requiring pony bottles, and I have rarely seen them being used. (I have done it myself on occasion, but in ways different from most people which I choose not to explain.) I would guess that regions where pony bottles are common are regions were safety bottles are hung for AOW dives.
On both the AOW deep dive and the Deep Dive specialty, I believe the most important aspect of the training is understanding how much faster you go through air at deeper depths. Consequently, training in that before the dive and careful monitoring of gas levels during the dive are critically important. If an AOW student of mine starts to run remotely low on air during the dive, both the student and I will know it. The student knows before the dive at what PSI he or she should be ascending to be safe, and that is when we ascend. One of the Deep Diver specialty dives requires an alternate air source, and I do have students hang a bottle and use it on that dive.
The dive charter's here act strictly as a taxi service. We are responsible for monitoring our air. I'm certaiinly not a lawyer, but hanging a saftey bottle seems like it would expose the boat owner to greater liability if poo should happen to impact the fan.