Your plugin’s UI (I shall use the term plugin in this article, but the same applies to your theme's UI) is one of the most important aspects of your plugin. It defines how people interact with it, how easy it is to use, and perhaps even how much they enjoy using it. This is the ultimate aim of your plugin: to make a particular task, or tasks, easier for your end user (in fact this is the seemingly forgotten aim of computers themselves). The UI should be attractive, but ultimately it should be functional. When deciding how to layout your plugin, you need to decide how to make your plugin easy to use - better yet, get feedback - this is essentially what WordPress is doing.