I think the issue is more complex than people who say: "never this" or "never that". A more useful conversation would center upon exactly what types of contingencies you wish to address.
If a diver is without a pony bottle (and diving with a buddy), then he SHOULD be leaving the bottom with enough gas to get him and his buddy to the surface in a safe and sound manner.
If a diver is very sure they are diving alone (i.e., solo with no pony) then the amount of gas required in the main tank to make a safe ascent is somewhere close to
only half the volume required above (possibly less if they are confident their emergency RMV will be controlled).
Now what about diving with a pony bottle and a buddy? I think things get a little more complicated.
IF a diver assumes there will only be ONE failure on a recreational dive, then he only needs to preserve enough gas in the primary tank to get ONE person to the surface. He also needs to preserve
only enough gas in the pony to get one person to the surface,
In this "one-failure" scenario, there are only two potential situations. The key issue is that the diver ASSUMES that only one of these two potential situations will occur (in other words no simultaneous failure).
The two situations are
- a failure (or loss) of the primary or pony bottle or...
- a buddy runs out of air and there is a need to donate.
Under this one failure scenario assumption,
then you only need to save enough air to get one person to the surface in each tank.
On the other hand, if you plan for TWO failures, meaning the diver experiences a scuba failure AND his buddy simultaneously has to receive air,
then you need to preserve enough air in each tank to get both people to the surface.
Is this second "double simultaneous failure" really something a recreational diver is planning for? I would think not. But some people might like to be extra conservative and plan for that situation. The worst case would be two people on one pony bottle and that presents a problem in that buddy breathing would be required (i.e., share one second stage). I don't see that being hugely practical because so few people are now trained to buddy breath.
So it makes sense to me how a person may assume a "single failure" and thus needs to preserve only HALF the normal amount of gas in both the pony and the primary.
Under this "single failure scenario" the presence of the pony bottle clearly allows the diver to safely drain down the main tank to a much lower pressure.
Defining the relevant emergency situation(s) being planned for seems far more useful than playing around with semantics like "pony bottle", "travel gas" ,"stage bottle" or "bail out bottle".