So you carry a pony on every dive to deal with an event having a probability of, what? Catastrophic failure where the reg fails shut and nobody is around to donate gas to you and you're too deep to do a CESA.
If you're diving in near-freezing water, with unreliable buddies, I guess the probability of such an event is significant enough that a redundant gas supply is advisable. Other than that, I suspect the probability is tiny--maybe similar to the probability of other events most divers don't have a plan for dealing with, like the boat abandoning you or, I dunno, a shark attack.
I plan for events for which the probability of occurrence or the severity of the consequences or a combination of those factors is high. With conservative gas planning, and even half decent buddy procedures, I don't see catastrophic reg failure exceeding that threshold.