I can think of several reason not to complicate the process and just get to the surface ASAP. I would want to have a good reason to take the bone away from the mean dog.
Under your scenario, I guess you are assuming that the pony bottle will be detached and handed to the victim so as to eliminate a physical connection between the victim and rescuer?
I would want to stay with the victim. If I start a rescue in a potentially deadly situation, I would want to see it through all the way to the surface - if practical.
It sounds like you want to hand off the pony bottle. what if the victim floats up too fast, then dumps too much air and then sinks super fast?
If he is separated from you, you could loose them and they will have no ability to use the power inflator to inflate their BC. Then they sink to the bottom and we know that many people forget to use oral infltion of the BC - at the least it adds significantly to task loading in a bad situation.
Under this scenario, they will be using one or two hands to hold the pony bottle, probably one hand holding the regulator in their mouth and then they have to use one hand to hold the second stage and another hand to work the inflator and they are holding the pony bottle between their knees on the bottom while they try to orally inflate a BC after they ran out of air once, then got separated and sunk down alone to the bottom while clutching a pony bottle? I can see lots of ways this ends badly.
Conversely, if I remain in contact with them, I can, to some extent, manage their Bc control errors (by offseting them with my own BC or manipulating theirs) and will always have the ability to use a power inflator.
I'm just not seeing the benefits of this regulator switch (or the assumed separation of the two divers which this presumably facilitates).
Sure. Like most diving procedures, there are tradeoffs, and you can always come up with scenarios that favor one or the other. And the decision on how to ascend would be based on a number of things, such as the specifics of who the OOA diver was, what their experience, training, and assessed mental state were at the time of rescue.
However, you are assuming that I was suggesting just handing off the bottle and leaving. I didn't say that. I just meant that if you are ascending it's better if you are on your own back gas.
There is nothing to prevent you from ascending with the OOA diver and doing all of the things that you mention, but a regulator isn't a good means of keeping two divers together. If the OOA diver is breathing off the pony, then no matter what happens he will have gas. If he is breathing off of your short primary hose, then there is a chance that you could get separated (surge, current, panic) and then he would have NO gas. Which is always going to be worse than any of your scenarios.
Finally, there is the possibility that he will become SO panicked that he will put you as the rescuer in danger. And in that case, it might be good to be able to break free without depriving him of his gas supply.
If I was diving OC and had to rescue an OOA diver with only one second stage and one pony bottle, I would do this:
1) Plug my primary into the OOA diver immediately.
2) Deploy my pony and breathe off of that.
3) Once the situation was stabilized, try to get the OOA diver to take the pony regulator
4) Unclip the pony and clip it on to one if his D-rings
5) Make a slow, safe ascent together with a safety stop as planned.
