1) 2nd Stage Unable to Deliver Air
When I was a beginner diver, I assumed a major reason for the octo was intended to handle your primary-regulator failing to deliver air. However, now I'm wondering how that's even "possible".
Just speculating:
- Mouthpiece is knocked off, or bitten off. I've heard of this happening, afterall, they're usually held on by zip-ties. Would it still be usable, just a giant pain? Obviously, you can breathe off the bubbles and purge button, but perhaps you could still directly breathe off it? (If it wasn't so cold, I'd try it. Maybe later....)
- Frozen regulator - I'd think the 1st stage would freeze long before the 2nd. I don't dive in areas cold enough to have a regulator freeze.
- Extreme Impact - A very strong impact could potentially break an internal component of the regulator. Though you might also be missing a few teeth. It seems more likely you'd cause a free-flow than break it in a way which prevents air-delivery.
- Unusable at beginning of dive - In the context of considering ditching your octo, it would be difficult to miss your primary/only 2nd stage not working at all while top-side.
I'm also speculating here, but perhaps at the time Octos became standard practice, 2nd stage failures (to deliver air) were more common, but a few minor chances to the design of 2nd stages have made those failures "impossible." I'd be curious if any of the regulator-gurus know anything about that?
2) When does an octo become nearly pointless?
The typical Sidemount configuration has 2 tanks, 2 first-stages, 2 second-stages, and no octo. This is generally more than adequate for air-share and redundancy.
What about a
properly used pony-bottle? For air-share, either the main-regulator or pony-regulator could be shared. There is some risk you might end up in a buddy-breathing scenario though if the pony isn't big enough for the other diver, or big enough to handle the extra air-consumption for yourself rescuing another diver.
For a solo-diver, so long as the pony is adequately sized, (IMO) the octo just become another hose to manage. Theoretically, if you "need" air from your primary-tank, your pony wasn't big enough. Assuming your pony works at the beginning of the dive, and regulators weren't run-over by a semi-truck, the chances of your pony failing during your ascent are probably much less than winning the lottery.