If you get 50 at surface, I believe it should be about 25 at 33 feet.
(PV/T = constant, so if you double your pressure by descending to 2 ATM depth, the air should occupy 1/2 the volume, in other words half the breaths)
If you're truly staying less than 30' depth, in the zone where you might not care much about a safety stop and/or just need it for a brief time to reach a buddy who you believe will be close by but perhaps just not paying attention at that time, a Spare Air might actually do the trick, used as you described. It does buy a bit of margin, albeit small.
NetDoc, a while back, mentioned he was thinking of testing one himself, just to see how it worked. I wonder if he had the opportunity.
I really did look at SA, and different ponies, before I bought mine. I knew I would be diving more in the 50 - 100' range, though, and wanted something suitable for that depth region. Personal preference -- I'd prefer one backup source, use it consistently, standardize, rather than, say, get an SA for really shallow stuff and then a pony for deeper.
No clue what the breathing is like on SA, the actual mechanics of employing it, whether you might tend to consume air slightly faster just due to the employment method. (Not to mention that you'll probably suck it down faster in a real OOA due to adrenaline.)
Best of luck! As usual, I can only describe my chosen equipment, why I selected it -- other kit may well be appropriate for other people.