I don't know what I would do if I ended up somewhere far from home without a phone.
In addition to my daily driver 7+, I keep the previous gen (6S+ presently) on me with a different carrier, and a 5C in my bag as an emergency spare.
Next year the 7+ will become the backup and the 7S+/8+ will go in my pocket, and I'll sell the 6S+.
I know people who live their lives without phones at all and I'm perfectly fine going days at a time without a phone.
Perhaps the guest mode should primarily be a means of preventing a thief from doing anything meaningful with a stolen phone. It should probably restrict the 'guest' to only using the phone mode and prevent the phone from being placed into airplane mode so that if it is stolen it can more likely be tracked and recovered.
I have actually put my phone into guest mode to lend it to people at the office.