I can see that DeV is not the target audience for such a GUI ;)

I think the difference ought to be clear as day. With a well thought-out GUI you will not have to search through tangled mazes of settings when editing themes that are not you're own. A particular setting will always be found in the same place. Besides you'll have (if the GUI's any good) additional little handy features like spin controls, drop-down lists or radio buttons for true/false, an Apply button that saves all changed files & recycles. Generally everything will be quicker to reach and easier to edit.

It should be noted that while I'm definitely in favor of such a change, I don't want it to replace the current way of doing it. I want to be able to edit config files directly should I need or want to. A GUI, for me at least, would be a time-saver. Something to rough out a theme with, or for making basic tweaks.