I'd say it's never reasonable for an employer to require an employee to provide equipment for doing the job at their own expense. Since the vast majority of people will have a smartphone it ends up not being an issue, but if someone chooses not to the employer should provide one (and can reasonably require that it isn't used for the employee's personal activity).
There are two separate issues here. One is about employer provided devices, where the likely requirement for segregation is security driven and personal use is likely to be a no-no
The second is requiring the use of that advice. Many of my colleagues are uneasy about having work software installed on personal devices, because of the way that tools like Intune blur boundaries when there are personal and corporate instances of software on a single device. But my employer is not unusual in having insisted on personal devices being used, enforcing it by the simple expedient of making it nigh on impossible to get a corporately funded device.
of course the issue behind this sort of stuff is the 'fake' self employment situations out there (esp in the gig economy) where people will be providing all their own kit but are technically not employed by the likes of the businesses they are exclusively carrying out work for
Not necessarily - there's a simple cost argument for large companies, even where they are only dealing with fully employed personnel
The argument about concert tickets raises a better case for a lot of people for how it's very hard to do your thing without a smartphone now. There are so many events, from small (Dice) to huge (Ticketmaster) that have only app based ticketing.
Precisely - I can choose to live like a hermit but, if I do, I will pay a significant cost in other ways. The choice is, in reality, not a choice that any individual can reasonably make