I know it's been covered by many people, but I wanted to try and get all the information in one place.
Firstly, let's look at the capacity of a spare air. 0.2 liters.
Let's be really, really generous, and assume you got your cylinder hot-filled so it'd cool down to 232, and filled your spare air right after they fill was done. You have a Spare Air filled to 250 bar.
At 0.2 liters, you therefore have 50 liters of air.
The "standard" SAC of a diver is 25 liters per minute. So, on the surface, you could expect to get 2 minutes of air from a Spare Air. Not too bad, is it..?
Now let's take it on a dive. And let's agree on this one thing before we start:
The purpose of a redundant air source is to allow you to ascend at a safe rate.
This is an important factor. You can surface from 20 meters down with NO air supply if you ascend fast enough. So obviously a Spare Air will get you up from 20 meters. The key point is that the Spare Air should get you from depth to the surface within safe ascent rates.
We'll use a 15m/min ascent rate, and ignore the "slow ascent from 6m to the surface" for now. That's actually faster than I like to go, but it's acceptable to most.
You're on a dive to 15m, you run out of air. You swap to your Spare Air and head up at 15m/min. Therefore it will take you one minute.
Will the Spare Air keep you alive?
Average depth on ascent: 7.5m
Average ambient pressure: 1.75 bar.
Gas consumption: 1.75x25 = 43.75L/min
So, you need 43 liters to get from the bottom to the surface, and you have 50. It's marginal, but you'd make it.
Now let's go to 20 meters.
Average depth on ascent: 10m
Average ambient pressure: 2bar
Gas consumption: 2x25 = 50L/min
At first glance, it looks like you'd just make it. But don't forget, it will take MORE than one minute to ascend from 20m at 15m/min. Your Spare Air will not get you to the surface from 20m.
Given that safe diving practices call for slower ascents than 15m/min, a much slower ascent from 6m to the surface, and several minutes for a safety stop at some point, it's self-evident that a Spare Air will never get you to the surface as safely as a scuba cylinder.
But what about my buddy, say the Spare Air brigade? I don't need to surface, just get to my buddy and his nice yellow octopus.
Well, this is true. So let's look at this:
10m down, Spare Air will last you one minute
20m down, Spare Air will last you forty seconds
30m down, Spare Air will last you thirty seconds
If you're at 10 meters, a Spare Air would hold enough to get you to the surface at a nice slow rate. But then, so would holding your breath.
At 20 meters, the idea of bolting for the surface is not attractive. You need an air supply. A Spare Air will give you forty seconds to get to your buddy and get his octopus.
I don't know about you, but I can hold my breath that long. And given that there's air in my BCD, I could last even longer than that without too much trouble.
In addition, the typical tidal volume of the lungs is five liters. That is, if you fill your lungs completely and then exhale completely, you will exhale five liters of gas.
At 20m, that tidal volume will require 15 liters of gas.
At 30m, that tidal volume is up to 20 liters.
Our Spare Air containing 50 liters can be emptied in the space of three breaths during a dive.
But the final nail in the Spare Air coffin is a much simpler bit of mathematics than the above calculations about air consumption and depth.
At my local dive shop, the Spare Air I could buy would set me back £180.
I bought a complete pony rig for £170. Less money, better solution.
I could also buy the top-of-the-range Twin Spare Air, for £310.
I can buy a twinset for £299. Less money, better solution.
The Spare Air is useful only on in the narrowest range of circumstances. For less money, you can get a better supply of air.
It's a mistake. Don't do it.
Finally, a link to the Diver test of the Spare Air, where it was found sadly wanting: Air Raising