I'm thinking of those cases where the reason for sharing the PIN is for it to be more convenient to do stuff¹ on somebody phone *for them* and usually under their request, but giving said pin requires a level of trust in the recipient that they won't use it to look at the phone at other times and/or for selfish / nefarious reasons such as spying on the other person.

(and it depends on the circumstances, but I'd consider “has asked for the PIN” a red flag that maybe they can't be trusted with it)

¹ like answering texts while the phone owner is driving, or checking a notification while the phone owner is in the bathroom, or the already mentioned managing the music, or navigation while traveling together, etc. basically cases where the other person is acting as a sort of voice based interface for the phone owner

If I were married maybe, in the interest of making logistical things easier if I died or got into a bad accident or something.

Not for "monitoring" though, I think it's unhealthy if there aren't still some private spaces in a relationship. If someone doesn't know me well enough to trust me in that way or obsessively needs access to every corner of my life, we shouldn't be in a relationship to begin with.

I'm not a relationship person, but if I were, I wouldn't want theirs, and they wouldn't get mine. Privacy is a vital part of a relationship. So is trust.

when we moved together we considered what to do and decided that we were both nerd enough to be able to set up a properly encrypted method of communication if we needed one, and that sharing the sysadmining was much more convenient.

And we also know the pin for each other's phone, but that's not really something important :D