A pony can be used to safely extend a dive.
Usually, it's called something other than a pony, and taken into consideration as part of your dive plan.
If one does take this route, I'd suggest reserving enough air to safely reach the surface with either tank you're carrying. For example, if you calculate (or test) that you need a 19cu bottle to safely surface in an emergency, and are carrying a 40cu, you'll want to reserve at least half of that tank until you start surfacing. That way you always retain redundancy at all times.
I dive SideMount, however many of my dives are shallow, so I'll often swap one of the bottles for a 19cu and treat it as a pony. In a way, the pony is simple redundancy, that I don't have to think about that much since I already tested it and know what it can handle. It's always 2500+ psi, which is plenty fine for 100ft for me. However, typically past about 70ft, I'll just switch to normal SideMount for extending the dive and a bigger reserve, while maintaining redundancy.
Redundancy is my real goal, and the pony bottle is just one way of getting there.
edit: The advantage of a "pony" that is mostly unused, is that it's redundancy that's always available. If you use your pony during a dive, it's not available for the next, unless or until you top it off again. It's also that people who are new to multiple bottles (or less disciplined) are less likely to use them in dangerous ways, if they simply don't use them as dive extenders.
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