lamont
Contributor
The use cases for a spare air are still highly narrow, somewhat implausible and are largely theoretical. If you screw up, if your buddy is close but not quite close enough, if you can reach them, if they can solve your problem in time, etc... And if you don't reach them or they can't solve the issue fast enough, then you run out of gas twice. A pony or doubles can solve a much broader set of problems at depth and lets you get out of the issue without having to bail directly to the surface.