a rec dive emergency SHTF tool.
I bought a 3.0 cu ft. Spare Air probably 12 years ago when I restarted diving after a long layoff, primarily because I had used them in the dunker/HEED trainers for helicopter evacuation and knew they worked in that confusing/chaotic situation, and also the "what if I had just one more breath?" marketing. I have had it hydro'd and serviced twice.
Switched to a 19 cu. ft. pony several years ago for my non-airline-travel recreational diving; backmounted doubles and appropriate stage and deco bottles for tech diving.
Resurrected the old Spare Air for a LOB trip on the Kona Aggressor in Hawaii a couple of weeks ago, primarily because I had two checked bags that were within 5 pounds of the maximum airline weight (two camera rigs mostly the culprit), and my 19 cu. ft. pony/reg/spg put me over the limit. Wore it for all the dives; nobody laughed. No one else had a pony or any redundant air supply on the LOB.
SHTF fool is an apt description.