Lea: The Unity design process (and how you can play a part in it)

> The use case is investing a lot of time in learning about and tweaking a system to their desires, either because they simply enjoy doing that, or to improve later user experience.

OK, so how does the extension system does not fit that use case? It provides a level of customization no other desktop ever provided. It allows editing everything the UI, the behavior of the UI, add new UI elements ... in 3.4 we now support sub classing gobjects / implementing them in JS which mean extensions can even do more stuff. The have almost the complete control of the UI.