I’d add one more design argument for UI chrome: when application controls should be persistent, it separates UI from the objects that UI acts upon. Many applications are adding UI controls that float on top of the main application view (for example, in both OS X and iOS, the new QuickTime floats the play/pause controls over the picture of the movie). Although this does maximize the size of the application view, and generally isn’t too unldrcovesabie, it’s really irritating because of the interference with the image. I don’t want to have to make all those extra taps to pause playback without seeing all those controls floating on top.