3. Failed first stage
Never heard of it happening. Could it? But then so could your pony, especially if you rarely - or never - use it.
You make some good points, but I'm calling this one a bit disingenuous: that is, claiming a redundant system is useless because it might fail as well as the system it's providing redundancy for. By that argument, we shouldn't carry spare tires on our cars because the spare might also get flat.
I will totally agree with you that there's no reason to carry a broken pony. But there is also no reason not to treat a pony system's valve, regs, and hoses the same as any other piece of kit, with regular maintenance and such, and I for one advocate ascending on the pony the last dive of the weekend/dive trip (when you would normally ascend, not to extend that last dive) just to stay familiar with the pony and to keep testing it out.
You mention first & second stages "never" failing, yet stories of freeze-ups and reg mouthpieces becoming separated from the second-stage body are all over this board. And besides the stages, there are hoses that come loose, break at the crimp, burst, and so on. One major LP hose problem and the primary tank can be drained in a fat hurry.
Another (minor) application for a pony - and an argument for keeping them filled with plain air and not backgas - is temporary MOD relief. A sudden downdraft takes you deeper than your nitrox MOD, or you drop your camera on the sand below your MOD. Now, some may say you won't oxtox taking a temporary dip below your MOD, but with an air-filled pony you don't even have to risk it (presuming you didn't drop it WAAAAY down there beyond pony capacity or - yikes - air MOD of course!).
You might not like ponies, but they are still by definition self-contained and redundant gas supplies, enough for a safe, normal ascent without use of the primary system at all when sized properly. Many of us find that redundancy alone well worth while, not to mention any other possible uses.
I also keep a spare tire in my car, filled with air as well. I've never needed it. Hope I never do. Doesn't stop me from carrying it, and no argument about its "worthlessness" will, either; although I wouldn't try to push one on someone who decided not to carry a spare. Your life, your decision.
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