I’m thinking of backcountry hunting applications personally. When I’ve been completely away from cellular service and put my phone on airplane mode and low brightness it goes for several days without a charge as a (not always on) gps and camera. Since these trips have usually been in groups of 3-4 the fragility of the phone is a smaller issue.

But I get what you’re saying, if you have 2 you have 1 and if you have 1 you have none. Some redundancy so nice and the iPhone as a backup to the inreach will be a likely set up.