I've already answered these earlier, but here goes.
I don't donate my pony bottle because it is my "Get out of trouble free" gas. I bring it to avoid a catastrophic disaster.
I do have an alternative plan if my buddy goes OOA. It's exactly the same as someone that doesn't bring a pony bottle. I hand over my primary, switch to my Air2 and ascend.
And do you mean to tell me that if your buddy went OOA and you donated your pony bottle, that you would continue diving? I guess you could continue diving until your buddy sucked down the air in the pony bottle and THEN you can share air to the surface. But I am a little more conservative. I don't consider extending a dive by any amount time worth the potential risks.
I also normally dive an Air2 and a pony bottle and I have no octopus. So I too would donate from my mouth and would switch to my Air2 and begin an immediate ascent. If, during the ascent, I felt that the remaining gas in the main tank might be getting low, then I would switch to my pony in an attempt to preserve the remaining supply in the main tank.
However, here is where we may differ... I don't have such a hard and fast rule about my pony bottle. It is another resource and I would share it if the primary tank was sucked dry, but then we would have to buddy breath sharing one regulator. Something I would like to avoid in a true emergency if possible.
As I think about it now, if I saw the main tank getting too low too fast from a panicked victim, I might decide to put them on the pony and put me back on the primary tank if I thought my lower breathing rate might allow me to make the surface with the main tank, even though it is low.
Getting the reg out of the victim's mouth and switching on ascent might be a challenge, but sharing one reg with someone in trouble would really scare me.
Also with respect to Boulder John's comment about using the pony to extend the dive.. I've done this a few times, but I am solo and using a small pony which is back mounted and I have no way to know for sure if it is filled with air, since I have no gauge that I can read once I submerge. The inability to not really know what is in the back mounted tank is a disadvantage which could be a huge problem.
So let's say, I find a really nice lobster at 100 feet and my main tank is down to 400 lbs. I am pretty sure if I remain calm I can make it to the surface with 400, but if I stay for just a minute or so, it might not be such a sure thing with just 250 psi (in the main tank).. So this is what I would do when I am (admittedly being an idiot).. I will preserve the 400 in the main tank, switch to the pony, try to grab the bug and get the hell out in a minute or so and go back on the primary for the ascent.
If the pony is empty or very low - then it will run out - (and when a small pony runs out- there is little warning and little ability to milk the air out of it because it is so small). But should this occur, I really don't have a problem because I can just instantly switch to my primary (abandon the lobster) and begin the ascent (although I have expended my redundancy shield on the bottom for a freaking lobster). I have a very good confidence that the gauge is correct and I have 400 lbs and that none of it leaked out in the minute or so I used to do the stupid human trick with the pony.
This is not the recommended method to use a pony bottle, but it is better than the alternative... Which should be obvious, but I will state it anyway: If you stay down and suck the primary to zero PSI and then switch to the pony (with no gauge on it) and find that it is empty (or runs out in 15 seconds)... well now you have a real problem.. Something you REALLY want to avoid. If your are going to be an idiot, at least be smart about it.
So yeah, I guess I understand how Boulder John views the pony as a resource to be managed on a dive.