As an "intermediate" (50 dives < 120 feet) diver diving the San Francisco wreck at Truk to a depth of 200 ft on regular air (there were a lot of very experienced divers with me on the Thorfin 1999) I had a pony bottle. I had the exact air integrated dive computer/regulator on the pony as on my main tank. If something happened to my main tank, then the pony bottle and pony dive computer would still keep track of my time, current depth (but not my decompression time which would have to be recorded from the main tank computer). Even though I had buddies, this would allow me some independence, and extra time to get to my buddies.
I think my reasoning is sound