I really want to use the white statusbar on my designs, but i couldn't find any apps that use this
It's a feature introduced in Android 5.0 with Material Design. Because it's set by a value named "colorPrimaryDark", maybe most apps just set it in dark tone. See this stackoverflow answer: https://stackoverflow.com/questions/22192291/how-to-change-the-status-bar-color-in-android
It may be forced by an app, but I doubt it. That is controlled by the OS which is also an option in the setting. The status bar color, along with the navBar, are themes for the OS.
Actually, no. There's a few apps I can think of that would benefit, though.
I haven't seen it used, but as far as I know it's a new feature in Marshmallow so it wouldn't be surprising if apps haven't started using it yet. It's in the official design spec, so I don't see a problem with using it.