Personally,
In as much as we set boundaries to our personal lives, I don't think our lives are so personal. As such, denying our partners the access to our phones doesn't speak good of that relationship. What do we then define openness or even transparency and how do we you'd agree with me that this one of the many things that characterizes loyalty. My opinions though.
This can help build trust and intimacy between partners, and can help ensure that both individuals are satisfied and fulfilled in the relationship.