72 hours is the bare minimum, IMHO. I think that some the logic behind that recommendation is because people assume that the delivery and distribution of supplies will go as planned- after all, the US Navy can have supplies to people hit by the 2005 tsunami. The other part is it is based on part on earthquake plans for SoCal, where there are a a lot of supplies prepositioned (in theory) and earthquake damage is often more localized and sporadic then the utter eraser that is a large storm. And to no small degree due to the experiences of wilderness SAR teams- 90%+ of the time, you're found in under 72 hours.

A more cynical part of my brain wonders if some it might becuase 72 hours out after a significant nuclear exchange, your hair and skin have fall off anyway.

As someone who's fairly rural, 120 hours is what I design for. Partially because it can take a day for a good ice storm to pass, then another day just get down towns cleaned up, I know that 72 hours just doesn't do it for me.