Consistency is important, but IMHO it should not be made a dogma. Some types of applications (maps, video/audio players, games) may just work better with a slightly different, non-standard UI, as they do not fit the normal desktop paradigm. In some sense, these applications create their own desktops abiding by their own set of rules dictated by specific usage case classes (i.e. "navigation", "media browsing", etc.).