Gecko has no superfluous capabilities. It just fetches Web pages and displays them. The rest of the stuff in the Gecko window is just there for decoration, and to show what the finished Mozilla browser will look like. The spartan functionality is used not because the developers have decided to remove these features. Everything else, such as bookmarks, history and so on are just add-ons that make your life easier, and are present in the full Mozilla program.
Gecko is just the layout engine.
The answer is that Gecko is not meant to be used only as a stand- alone program, nor is it meant to be just part of the finished browser. Gecko is an embeddable component . It can be used by any application developer to add Web browsing functionality to their application. But once again, I'm getting carried away. All in all, pretty neat.