A pony bottle is the only true safe solution when it comes to a redundant backup. I own a 9cf pony bottle and usually use it on wreck dives. It is assuring to know that I have a redundant backup in the case of a tank valve or first stage failure, hose rupture, uncontrollable free flow of air, tank valve o-ring failure, overfill pressure burst disk failure, etc. As for the spare air, you are correct. 30 breaths may not be enough to keep you out of the chamber during an emergency ascent, but a conscious DCI diver in the water is much better that an unconscious DCI diver. I find that the benefit of the SpareAir is to buy you time to either get to your dive buddy or the surface. Many times, your buddy may be a short distance from you and the spare air would give you that extra minute to get a buddy’s attention. An added benefit of the spare air is the ability to hand it off to a diver in an emergency. As stated on the SpareAir website, the product is designed for the "if I only had one more breath" situations.