wow..I don't think this was a thread to bash how people dive certain rigs. This was supposed to provide pros and cons of spare air and pony bottles. Anyone can look at anybody and criticize their rig setup. We are all different, and therefore dive different. What works for me, won't work for someone else, and visa versa. If I want to dive solo, YOU can't tell me I can't or shouldn't. And if you need to have a friend that you've known all your life as a dive buddy to be able to dive...so be it. I not gonna say boo. I wish people would get over themselves. Let people dive how they want, 'cause thats what you yourself are doing.
As is often the case, I've apparently failed to properly convey what I was trying to say. Adobo did a very nice job of clarifying, but I'll take one more stab at it because, well, I'm a glutton for punishment. It wasn't my intent to bash anybody's gear config or dive style choice. if you want to dive solo and you're aware of, and comfortable with, the associated risks then by all means have at it. No skin off my nose.
My point in a far more general sense was this- if you examine nearly all dive accidents, they are rarely the result of one thing gone wrong. They are nearly always the result of 3 or 4 (or more) minor things that went overlooked or ignored until, by some unfortunate coincidence, they came together to create a bad situation. These seemingly minor things often get overlooked because of a sense of complacency, or a casual dismissal of their potential impact, or a total lack of awareness that they are even issues. They are, after all, just minor annoyances and 99.9999999% of the time they result in nothing..... until that one time that they result in something.
My opinion (and it's just that, so do with it whatever you wish) is that something like a pony bottle can often create a false sense of security that fosters the oversight of these seemingly minor things and inhibits the diligence in dive planning and plan execution that's critical to safe diving. "Why check the SPG regularly since I've got a pony bottle? I'm busy taking pictures, after all." That's fine, until that one time that you forgot to check if the pony was full, or if your buddy, who has no pony and is also busy taking pictures and hasn't been checking their SPG is also OOA.
What I've endeavored to say is that you simply need to ask yourself why you think you need a pony, and what problem you're trying to solve. If you need redundancy because you're solo diving- that's fine. If you need it in case "something" goes wrong- fix the stuff that can go wrong first.