it all depends on how deep you're going, if you're spearfishing in shallow water, lets say 30 feet or so, and you only need help getting to the surface, then a spare air would be fine. My instructor teaches their use, and I had one that I won in an online draw. I used it in a local quarry with my cousin when he was checking out and setting up new gear, I easily stayed at 15 feet with him for 2 or 3 minutes, doing bubble checks and checking straps. Now that being said, I HAD one. I sold it because the vast majority of my diving is in Lake Erie, with the shallowest decent wreck being 70 feet deep, and I deemed it as useless at those depths, and I now dive doubles for redundancy, but used to carry my instructors 40cf pony when I was still in singles on those deeper dives. I think having totally redundant air source is a great idea, you just have to be realistic about the depth. Spare air claims 100 foot operating depth for their bigger unit, and it might let you bolt to the surface from there, but I would prefer the ability to make a controlled acent, or have ample redundant air to swim to my buddy and make a controlled acent together, with him as your backup now, and I dont consider a Spare air suitable at depths much beyond 30 feet.
