A pony (as a pure emergency air source) is almost exclusively a recreational diving tool. In the tech realm, gas planning is much more complex and you are right, a pony is insufficient for their needs.
One variation of gas planning is the rule of thirds. One third there, one third back, one third in reserve. With doubles on an isolator, if one post fails at the worst time (turn point) each of the two tanks is 2/3 full... Isolate, and you still have gas to get back. Or buddy has total catastrophic gas loss, he gets your long hose, you're "reserve" was equal to worst case return needs, so you both still have gas to get back.
This is oversimplified examples... I don't have sufficient training to go further with that.
For my non-overhead NDL diving, my 19 is actually overkill.... For tech it would be laughably insufficient.