Another vote for "Spare air is dumb." I say that, as someone who owns a spare-air.
The TLDR for anyone who isn't informed on the subject yet, spare-airs are far too small to be of much use. To bail out at 100 to 130ft, and do a safety stop, you'll need approximately a 19cu pony bottle. To do the same, and skip or shorten your safety-stop, you'll need at least 13cu. 6cu might (or might not) barely last an emergency ascent. 3cu or 1.7cu, and you might as well not carry it.
You can get a pony-bottle and cheap regulators for about the same price as a spare-air. You don't need anything too fancy for the regulators, just something that works. Plus, if you decided to do a brief shore-dive or something along those lines, the pony-bottle could actually work, whereas the spare air you'll find yourself running out in a shockingly short amount of time. Spare-air's don't really have a pressure gauge (technically, there's kinda one, but it's not readable).