If the only need for such "turning off" is just to hide one's username on top but it doesn't cut across previous posts made, I don't see its usefulness. If anyone doesn't feel comfortable people peeping into their devices while they open them in open space, then it's only reasonable not to open it at all when in such place(s). Sometimes, we throw ourselves into unnecessary secrecy by hiding things that aren't even relevant hiding. That's the way I see this approach.