This does assumes that the replacement phone in question is unlocked so can work on the other SIM card, but is certainly something that many people do quite often.

Another scenario where this causes issues that a login equivalent can improve: imagine losing your phone including SIM card. This then causes a full interruption of all services tied to that phone number including actual phone service, and SMS, until the SIM is replaced. This usually takes hours/days depending on your location or carrier.

The other big difference in just swapping out sim cards is the need for a full phone reset and the interruption to the service of whomever you're borrowing the phone!

I want phone-as-easy-as-email. I logon and immediately calls are routed to that device and I can pick up voicemail, make calls and send texts. If I have another phone account I can use the same handset to log into that account at the same time and all calls to that number get routed to the device as well. If the device has radio hardware (such as phone handset) it uses the radio network otherwise it uses wifi or whatever other network comms that device has. When I logout it all stops.

As a customer that's the interface to 'phone' that I want. Phone is soft, not hard.

SIMs are secure(ish). Hand typed passwords are not (no one want to memorize and type a 20 character password formed from a mix of upper/lower/numbers/symbols).

(yes, sims are hacked and cost carriers billions. hacking sw passwords is a lot easier).