I have just read through this whole thread because I too am interested in a pony bottle. I got the 6 ft*3 Spare Air and I still think it has a purpose. Not for deco accents from 150 ft, maybe not even from 60 ft*3, but I got it to get to my insta-buddy more than to get to the surface.
I do most of my diving with insta-buddies. Some are very skilled and wonderful. Others are divers in the same ocean. That said, I wanted something I could easily pack, and easily fill at my destination. Given that you have to remove the valves, most shops, if they know, will require a new VIP. Since I can fill the little bottle off my main tank (I usually do this the first dive, which is usually a buoyancy test for the group, and we don't go deep or long, the missing 6 ft*3 from my 80 (or 77) is not a problem. But, if I had to get a new VIP that can get expensive, and time consuming. I'm not even sure how I would do that on boat dives where they bring just the number of tanks you bought on the boat. No compressor. So it would be a special trip to fill (or so went my reasoning).
Ok, now I am thinking I may be doing more solo diving (often with another diver in the same ocean) so I want a bail out bottle. If I decide to go tec, I'll get a deco bottle. I'm very much leaning towards the 19 ft*3, but since there is so much discussion on what one really needs, I think I'll borrow one and try it out from 80-90 feet. Granted, I'll probably be more relaxed than in a real OOA situation, but if I don't make it to the surface in a leisurely accent with a 5 min safety stop, as some believe, then I know I won't in a true OOA situation. If I get there with 1/2 a tank, it will probably work. I'll just have to make some judgments, but I think by diving it, I will make smarter ones. They won't be based upon resale value, or upon potential use for something I'm not buying it for now. It will be based on what I need to do the instant job, with some reserve.
And, I'm really hoping that the 19 ft*3 will work, since I believe I can build and trans-fill that from an 80 ft*3, and still have enough left in the 80 to do an easy dive (and bypass the extra VIP). I haven't run the calculations yet. I do know that I'd end up with less than 2000 psi in both with a 40 ft*3 tank trans-filled. In addition, I'm not sure I'd pack (due to weight and size constraints) a 30 or 40 ft*3 pony on my recreational dives. Especially if the airlines follow suit and limit you to one checked bag (and on some airlines, that bag is limited to 30 pounds).
Is my reasoning flawed?