The original AP Diving 'Buddy' Jackets came with a 1 litre bottle attached. They where designed so you could breath off the Jacket if required. This approach has fallen out of fashion (not to mention that you needed to clean the bag regularly to avoid catching something nasty), as has the Air2.
When I started, twin cylinder rigs where seldom seen. The small group doing deep diving, or wanting redundancy where using Pony's, that made you a small subset. Over time it became noticeable that the number using pony's increased, at the same time, many of us switched to twins. Then twins with decompression stages, then CCR with bailout.
(In fairness, twins where used back in the day - twin hose regulators - look at the Jacque Cousteau films.)
A lot of the change reflected the progressively deeper diving that was being done, not the extreme depths of the 1%, but the fact that larger and larger numbers where doing 50m diving, or doing mid range dives with long decompression penalties. Nitrox came onto the scene, which was adopted for decompression, before it was used on the shallower dives. Then Trimix.
Even when diving twins, we often used to switch back to singles with pony's for RIB diving. A single with pony was ideal for sub 35m diving, or wreck diving in the confined space of a RIB.
Much over 30m, then twins are a lot more optimal.
As someone also said, a Pony is like a Bailout on CCR, never to be used to extend the dive, used for emergency only.
There should always be sufficient gas to get you off the bottom and to complete any decompression penalty you have acquired. Thats how we used them.
Once you switch to using a Pony for decompression gas then its a stage, not a Pony.
When Nitrox became available, all of us on Twinsets dug our old ponies out and filled them with 50% and bolted them the side. Eventually we went to larger stages, then learnt our lessons and switch to Aluminium stages, much better than steel stages.
I think one of the reasons that few American divers use ponies and have such a negative attitude towards them, is because of the attitude to dives involving decompression.
It goes back to the fact that dives involving decompression are seen as some form of special diving.
In the UK, dives involving decompression are seen as normal recreational diving. As such, most UK divers do some diving with some amount of staged decompression. Because of this decompression diving and redundancy of breathing source is taught within the core diving program (as is Nitrox), and a high proportion of regular UK divers look to add some form of redundancy. Like a lot of divers, many don't want to do deep dives or large amounts of decompression, so twins are not desirable. But a small light cylinder, they can add to their single. With sufficient gas to get them safely back to the surface, is a sensible addition to their recreational diving kit.
In addition, we don't always have clear blue water, and wreck penetration is often part of UK diving (some times accidentally in poor vis'). So redundancy is a sensible addition.
So the simple answer to the original question, is YES. There is often a valid reason for a Pony Bottle.
