Interesting discussion. Besides the phone, browser and email apps, most of the apps on my iPhone work off-line. The map works and the GPS will plot your position without cell or wifi connectivity.

The latest smartphones come with quad and octa-core processors and have a lot of processing power. Choose your apps well and it can be a valuable tool. Indispensable? Probably not, but still valuable. All it takes is a compatible solar system and it can provide for lots of use. I have a case around mine that does a good job protecting it from accidental drops onto concrete.

As always, YMMV.