Some iOS7 animations such as starting text input (e.g. try clicking on Search field in the Contacts app) require animation to go through-under the status bar (bat with signal level and clock), parts of screen "fly out" though-under the bar while Search field is being moved to top.

So statusbar is to be a part of the app window then and you should be able to paint/animate under it. Ideally the bar should be the optional as some apps need exactly whole screen.