Web Monkey:Pony bottles seem to be a hot topic around here, but I've never seen a valid reason how an extra 19 or 30 Cu Ft of air and completely redundant regulator could be bad. Maybe it's just a macho thing.
The rest of the arguments against using it are mostly equipment configuration issues. For example, if you sling it in front instead of behind you (several companies make clips to hold it), turning it on or off or giving it to another diver becomes trivial. Also, although it does throw your weighting off, it only does this for a few minutes until you figure out how much you're off by and shift your weights around as needed.
Terry
I agree with much of what you say. Just call your pony bottle a slung "stage bottle" and you'll be fine around these waters
The reason why I usually frown on ponies is, that people tend to ignore them: fill them once, never verify them, fit a crappy reg on it and then not do propper gas-planning, assuming that "I have my pony, so breathing the backgas down to 2 bar is OK"....that's no good, IMO. It's even dangerous, because a "pony" may be percieved as a security blanket and other good diving pratices go out the window. It's easy to say "I won't do that", but many many divers still do.....
Yes, I do carry a "slung" bottle (or more...) when I dive, in particular if it'll be a deco dive. I keep the reg on my "slung" bottle in as good as condition as my primaries. And, indeed, they're the same high-performing, expensive regs. If it can safe my life to spend 200 Eur extra per stage-reg...well....
If it's just a "recreational" dive with no to moderate deco obligation, I keep EANx32 in a single "slung" bottle. I calculate my gas-plan to be done with backgas only. I also calculate from which point of the dive I would be able to switch to the "slung" bottle and have enough gas to finish the dive. I then dive the plan, which means that I would be able to complete the dive entirely on backgas, with a suitable reserve. And the volume in the pony still intact.
For more serious deco, the very same bottle is perfect for EANx85-100 for the 6m and above stops.
The point to all this is, that I am much in favor of redundant gas supply. However it takes training to know how to use it correctly, and it takes a lot of care and consideration to not let the "pony" be a false security. And it is more gear to verify, maintain and be familiar with. Furthermore, not very many OW divers are familiar with gas-planning. Quick, how many knew what "SAC" meant right out of OW-class? How many knew what YOUR SAC was right out of OW class? Not many, I bet.....unless gas-planning is taught and praticed, it's difficult to quantify the "security" offered by a pony or any other redundant source of breathing gas.
Of course, the ultimate "pony setup" is twin tanks with isolator