Elemental would benefit from a skinnable interface that allows custom menus and entirely new menus to be created without programming changes.

I don't know the internals of Elemental's engine. Maybe this is already possible.

There are many reasons to do this in a game. One of them is that it greatly extends the possibilities available to modders and even the real development team down the line if the interface is easily editable. This is stardock, so we all probably know that skinnability is really important to some of us, but it's not just about being able to change the background to pretty colours. Being able to modify the interface is key to being able to create many kinds of systems, including shifting the kind of game you're playing from (for instance) 4X strategy to Dungeon-crawling Adventurer.

NWN2 (which by itself was not a perfect game) did try to innovate with a very strong, data-driven interface system. It enabled users to implement whole new features that simply weren't possible in the original system. Modders created everything from new menu bars, Dungeon Mastering tools, to readable books and minigames just from having access to the interface.

It also enabled the developers to revist the game in subsequent expansions and readily create brand new interfaces without signifigant work.

For example, compare the below images.

The first image is the base game with no modifications. In the second image, the developers have created an overland map system and integrated it into it a classic, story-telling style of gameplay. The game style has changed from "Diablo point and click" to "Livingstone's choose your own adventure." You can see a conversation window giving the adventurers options for dealing with approaching Kobolds. You can also see a further window where the skill roll to spot the kobolds is made. Not only is a static map shown, but through clever usage of lighting and scaling, a large overall area is displayed that gives a feeling to the location the adventurers are in. Supplied are full images to get a better idea of what is being shown here:

http://journals.rpgnexus.net/content/imgs/NWN2_XML-Driven-Interface_01.jpg
http://journals.rpgnexus.net/content/imgs/NWN2_XML-Driven-Interface_02.jpg

It is not only possible to create unique systems, but entirely new sub-games, or minigames, when the right approach is taken. Civilization 4's approach led to the creation of an in-game cardgame that players could play, that was integrated into the popular FfH 4. You can't even tell that's been done using Civ4 - that's how much power being able to modify the interface has.

These are a few good examples of the kind of gameplay variety modders can explore when they have access to a robust skinning system.