[A pay-as-you-go plan] is cheap, but effectively worthless for anything other than a rare quick phone call or text message
I use it for exactly that. Longer voice calls wait until I arrive at home, where we have a phone on a different plan with unlimited minutes and zero texts. Longer text conversations wait until I arrive at home or at a hotspot, where I use Internet-based text chat or email.
and if it's actually a smartphone, then it's a waste of resources altogether.
I disagree. Even without cellular data, my Android phone is no more "a waste of resources" than an iPod touch. On this 5-inch tablet, I can still access locally stored information anywhere and connect to the Internet at any hotspot.
If you carry a device for emergencies only
I carry it not only for emergencies but also for the sort of urgencies for which one would have used a payphone in previous decades. The most common is calling home to arrange a ride after the city buses have stopped running for the night or for the weekend.