I don't know about you, but my old phones struggle with room tempurature air, I'd hate to see how they do in an attic. That with the battery (most can't run without one because of spikes in the current draw) is a bit of a fire hazard.

Imagine a mobile web server. For events, geocaching, emergency services, and other on-demand situations. It could serve local wifi to serve its pages.

There a bunch of LAMP apps on Android. You just need a phone that open enough to let you 'share' a connections. Otherwise you would just be better off with RPi setup as an access point.

https://play.google.com/store/apps/details?id=com.esminis.se...

Running on solar, perhaps? Free computation when the sun shines!

Or botnets