As a paddler I can see a lot of potential upside to having a small scuba bottle while paddling. As a diver I think Spare Air is a lousy back up plan. There are much better (and cheaper) options available. Sitting at home and reading this post a breath is probably about 1/2 liter and you breath about 12 times per minute, but if you need to use your spare air while paddling you probably won't be taking average breaths. Instead of 90 breaths from the available 45 liters, the 30 advertised "surface breaths" is probably more realistic. If you can manage to breath slowly that could give you 3 minutes to solve whatever problem you're having. That's not a lot of time for a bad pin or a terminal hole, but it's a lot longer than only having the one breath you took on the way in.
The big problem I see with a spare air is that because the mouthpiece and regulator are integrated with the tank you can't attach the tank to your PFD and just put a small regulator in your mouth. Holding the whole thing in your mouth while practicing rolls in calm water might be fine, but what's going to happen in the real world? If you're pinned with your back to the current or in mild current you may be able to hold it with your mouth, but more likely you'd need to have at least one hand on it to make sure it isn't ripped away by the current. Having only one hand to solve whatever problem you're experiencing isn't going to make things a whole lot easier.
If I was going to carry some type of scuba bottle I'd probably use a 6 CF pony bottle. That would offer more than 3 times as much air in a package that isn't all that much bigger. More importantly it would let me attach the pony bottle to myself by any of several methods and I would only need to hold the regulator in my mouth. That gives me a far better chance of having both hands free, and if the regulator is pulled from my mouth I can get it back. Some divers use a bungee cord to hold their spare regulator around their neck, and can reach if just by leaning their head forward. Not that you'd have to keep it there while paddling, but a similar bungee would increase your chance of keeping both hands free while still breathing. If you can still return the Spare air I'd suggest doing so.
I'll also point out that if you're not a diver you should learn some basic diving physics and physiology if you intend to breathe compressed air while under water. If your chest is underwater the pressure will be more than the air pressure at the surface, so the air you inhale will still be compressed relative to the surface. If you inhale while you're underwater and then surface while holding your breath the air in your lungs will expand. Ascending as little as 4 feet while holding your breath can cause an arterial gas embolism by forcing the expanding air into your bloodstream, or it can rupture your alveoli like so many millions of overinflated balloons. It would rather suck to escape a pin that might have drowned you only to die unpleasantly as soon as you're out of the river or spend the rest of your life as a brain damaged veggie.