The whole reason for a pony bottle is to have air for an
OOA situation or equipment failure, end of story!
It’s NOT as I have seen to be used to extend bottom time
or to compensate for bad SAC or smoking.
In the old days we would leave a 72 on the sand or deck
of a wreck for emergency’s, the problem with that plan in a panic is the OOA
diver would swim right past it and bolt to the surface which rarely ends well. Hence
the need for a smaller bottle you could keep with you.