One possible use of this new feature is to silence calls when sitting in a meeting, watching a movie in a cinema or being in a live show like theater or opera. But Do not disturb only works when iPhone is in standby ("locked"). What is if the user is in a cinema and wants to take a note? Maybe he wants to set an reminder or something else. And as he does what ever he has to do, the iPhone is on and that means Do not disturb is off without notice! The moon on the menubar is still there but calls wont be silenced! That could bring you in trouble.
I think this new feature needs an update. The moon should go away to indicate that D.n.d. is off while iPhone is on or calls should be silenced as expected.