Mobile phones will never be shared by people who don't already share them. This is a convenience functionality, not something that would change how you use the device - from the exact reason you said: mobile phones are extensions of identities. This feature is something you would use when your own phone is out of reach (e.g. on a shelf in the living room) and your wife's phone is with you in the kitchen.

True, it will not exactly boost sales, but it will not decrease them. It will make some people more likely to recommend Apple. Everyone will still have their own phone.

You can overcome the risk of decreasing the likelihood of creating personal attachment by letting the foreign user log in to a de-personalized (no custom wallpaper and so on) space and use a limited subset of functionality, e.g. a browser, contact list, the Apple messenger app and a phone app (that would call from your own number/phone over VoIP); this functionality would be available only when both phones are connected to the same wifi.