One thing that consistently comes up as a reason for using a pony is buddy separation. Being alone underwater does not mean out of air. It does not mean you are suddenly in a panic situation. If someone feels that they are unable to complete a dive without another diver next to them, perhaps more training is needed. It takes a catastrophic loss of air to need a pony. A blown hose, free flowing reg or extruded O-ring is an emergency situation that requires an immediate ascent, however, in those situations, you would still be able to breathe while ascending. Only a complete first stage shut down, which would be extremely rare, especially with maintained gear is the only time a pony would be absolutely necessary.
Pushing ndl with a single tank or putting yourself in potentially hazardous situations at depth are easily avoidable, yet we keep hearing posters claim that is why they need a pony.