The primary reason I carry the stony bottle is not because I'm converned about an OOA situation. I carry it and use it on every dive even if I never take a single breath from it which is most dives. Because I've got a full 19cf or 30cf cylinder, I draw my main tank lower than "normal", thus extending almost every dive. If the dive is particularly interesting, I will draw my main tank to around 700 psi (varies depending on depth) switch to the stony bottle, breathe it down to say around 1000 psi then switch back to the primary to ascend, and typically be on the boat with less than 200 psi.
A bonus of carrying the bottle is of course in the event of needing more time at depth to solve a problem such as an entanglement, or a gear issue, or accidently ending up off the dive site and under an area where I'm hearing heavy boat traffic and a slow cautious ascent is needed.
If you dive off of other commercial dive boats and especially if the boat is live, you are going to end up far from the boat at times and have a bit of a wait. A signaling device makes it much easier for the boat to spot you, and is much safer when it comes to other boats seeing you and avoiding you especially if deployed from depth. An audio signaling device can also be useful to get the attention of a boat when it can't locate you.
I'll wager to guess you don't carry a PLB either. Divers do get left behind, lost, swept away in random directions from current.. It's a tough ocean out there and conditions can be unpredictable.