The only reason, I believe, that a couple would ever think of having privacy with their mobile devices would be because there is some form of distrust existing between them.
I believe, naturally, that both spouses should have nothing to do with the other's mobile device, unless there is an actual need to. For example, one may need to make a call as they may not have the airtime to do so.
For what purpose should "1 + 1 = 1" be for when there is a thing like "the necessity to access the other's device" to validate trust?
But really, it all comes down to the decisions a couple makes in their marriage. They could choose to have this level of privacy, and they could as well choose not to. In the end, it is whatever that works for them that matters.
Well said, somehow you've managed to hit all the keys points in this comment. There's nothing to hide but it doesn't mean one should go through the others phone. In the end different couples will have different levels of privacy, thanks for stopping by.
!PIZZA