I tend to agree with the other posters on this question. While the spare air seems like a good idea, generally speaking if you have an OOA emergency, your redundant air supply should be as big as you can conviently carry. Nothing has given me more peace of mind while diving than having a pony bottle strapped to the side of my primary air supply. I am sure that my pony is bigger than I'll ever need. I purchased a 30ft bottle, a reliable regulator, and a mounting system for about $375. The mounting system for a 13ft bottle is the same as the 30ft. So, once I decided to strap on the extra tank, it made sense to me to get the larger bottle just in case.
Basically what I'm trying to say is.... I beleive EVERYONE should dive with a redunant air source. The Spare Air is a good option, but a pony bottle can do much more for basically the same price.