Pony bottles are a security blanket, not a solution. Some divers feel better carrying them but they aren't the best solution to
any problem and they create new problems of their own. One complaint I have about them is that I see divers extending beyond what they have planned for, figuring that they can cut their margins closer since they have their little back-up bottle.QUOTE]
Ha ha ha - hey - doubles are security blankets for divers who can't hold their breath.
The redundancy in doubles extends to a properly configured/utilized pony. If a diver uses and plans pony gas as part of his/her dive plan they are not using the pony correctly. The pony should never be touched except in case of emergency. If a pony is a security blanket, trust in a buddy to solve your gas/equipment problems is blind faith. It's like saying you do not need to wear a seatbelt because your friend is a good driver. I just assume have my own safety solutions in case another human being cannot provide them for me. How often is your buddy right beside you, within arms reach to his/her regulator, that you could grab with your eyes closed, during every second of a drive from splash to surface? I have never seen, heard about, or read about buddies who can offer this precise redundant-regulator-to-mouth availability as I described above offered by a pony (or doubles).
I dive doubles with isolator. I only use a single with pony when doing shore dives (winter) or while on vacation. It does not make sense to me to give up the redundancy I have in my doubles when I am diving in a situation calling for a single.
I agree that H/Y valves are a better safety solution compared to a standard single valve.
--Matt