There are several comments from people that I have come to have tremendous respect for, that don't believe that independent air sources are appropriate in "basic" diving. IMO, while buddy-diving is the "basic" standard, the reality is more complex and peoples views are colored by their environment.
Personal, anecdotal results from my admittedly very limited experience ... in about 15% of 'buddy' dives I've been effectively a solo diver.
- Buddy just swam off and never looked back (actually that time I was having issues, fortunately only equalizing on a sawtooth tour dive.)
- Buddy separation in the murk several times, once surprisingly, a couple not. All to easy in the fall at Dutch where viz in the training area is terrible at baseline and goes to hell when it is packed like a Manhattan sidewalk at rush hour.
- Got tired of chasing an otherwise competent instabuddy flying high in the Coz current and chasing underwater squirrels ... (I did not ignore, but rather monitored her closely and positioned myself to be available to respond quickly.)
- Lake diving this last Monday - my buddy was bumping into me and I couldn't even see him (Ironic WRT this discussion - I was not actually carrying my pony since it was a < 20' bottom, CESA was a practical, "safe" option)
So now I carry a pony which gives me full redundancy of gas, valve, & 1st/hose/2nd. And additional time to sort out a problem for myself or others.
I have a 19cu.ft. for air travel or shallow dives and a 40cu.ft. for deeper stuff. My pony reg set is from the same family/performance as my main regs so acts as a backup/parts when traveling - I'd rather rent/borrow a reg for pony use. The 19cu.ft. was not that big a deal going to Cozumel and back - Bled it and removed the valve, rubber banded a plastic bag over it, shoved it checked-baggage. I asked before booking and the shop had no issue filling it for me.