What is the purpose of carrying a pony bottle? It can have two basic purposes: One is self-rescue in the event of being out of gas yourself, and the other is a separate source from your own gas, that you can share with or hand off to someone else who has run out of gas.
How likely is it that you will be out of gas? If you have some tools to do some basic dive planning, and you are responsible about monitoring your gauges, the only way you will run out of gas underwater is by having a catastrophic gear failure. Luckily, those failures are extremely rare. So I think it is fair to say that a responsible diver using good planning tools is very unlikely to be out of gas.
Now, if you are diving with another diver who is responsible about his dive planning and gas monitoring, and both of you plan to maintain rock bottom reserves, then in this very unlikely event of running out of gas, you will have a diver close at hand who you KNOW has enough gas for both of you to get to the surface. If you are diving the GUE system, you will not only make it to the surface, you will be able to execute an ascent which is basically NORMAL, so it puts you at no increased risk of any other bad outcome from your gear failure.
I bought this as a legitimate approach to the risk almost ten years ago, and I still believe it. The biggest reason for carrying a pony is not trusting your buddy to have enough gas for the dive, or to maintain reserves for you -- or to remain where you can reach him, or to be competent to execute an air-sharing ascent. This is the solo diver mindset, where someone has decided that buddies are unreliable and therefore it is necessary to provide self-rescue. In my world, buddies ARE reliable; on the occasions where I dive with someone (for example, a brand new diver) where I'm unsure of that, I either keep the dive so easy that I figure I can self-rescue at any point, or I dive doubles, or best of all, I bring along a third teammate who is MY bailout.
Dive with good folks and dive conservatively, and you don't need to carry another bottle and regulator on simple open water dives. Dive deeper or add complexity, you might want some redundancy -- but at that point, why not go for a small set of doubles? You're carrying an extra tank either way, but with doubles, you have so many more options.
How likely is it that you will be out of gas? If you have some tools to do some basic dive planning, and you are responsible about monitoring your gauges, the only way you will run out of gas underwater is by having a catastrophic gear failure. Luckily, those failures are extremely rare. So I think it is fair to say that a responsible diver using good planning tools is very unlikely to be out of gas.
Now, if you are diving with another diver who is responsible about his dive planning and gas monitoring, and both of you plan to maintain rock bottom reserves, then in this very unlikely event of running out of gas, you will have a diver close at hand who you KNOW has enough gas for both of you to get to the surface. If you are diving the GUE system, you will not only make it to the surface, you will be able to execute an ascent which is basically NORMAL, so it puts you at no increased risk of any other bad outcome from your gear failure.
I bought this as a legitimate approach to the risk almost ten years ago, and I still believe it. The biggest reason for carrying a pony is not trusting your buddy to have enough gas for the dive, or to maintain reserves for you -- or to remain where you can reach him, or to be competent to execute an air-sharing ascent. This is the solo diver mindset, where someone has decided that buddies are unreliable and therefore it is necessary to provide self-rescue. In my world, buddies ARE reliable; on the occasions where I dive with someone (for example, a brand new diver) where I'm unsure of that, I either keep the dive so easy that I figure I can self-rescue at any point, or I dive doubles, or best of all, I bring along a third teammate who is MY bailout.
Dive with good folks and dive conservatively, and you don't need to carry another bottle and regulator on simple open water dives. Dive deeper or add complexity, you might want some redundancy -- but at that point, why not go for a small set of doubles? You're carrying an extra tank either way, but with doubles, you have so many more options.