The human brain is naturally attracted to looking at pictures of human faces. When you're eye gets to the face, it likes to see it in focus, even if it's not the subject of the photo. It's not a hard and fast rule, but it's something viewers will notice, so if you're going to throw blur on that face, you need to have a good reason for doing so. For instance, maybe a wedding shooter wants to show off a the gown while the bride looks in the mirror. Will it work? Sometimes. But, it's more of a challenge to pry the viewers eye away from the face and onto the primary subject.