Personally, I prefer INI files anyway (well, INI-like at least). Yeah, call me old-fashioned, but they're a lot easier to deal with. XML is good when you need tree-style nesting, but most of the time you don't, really (and even then, those using XML more often than not abuse it resulting in ridiculously complex formats for no real reason).
Added systems for localization AND fairly comprehensive theming in a single night for our next release... using ini files. Still need a good sit-down or two to link remaining text, gui elements, etc to these systems, but the functionality itself is complete and works great.
Honestly I've been paranoid that the design choice was naive or missing something critical because it was so darn simple and ini files are all but unheard of these days - but it works like a charm! Not to mention it takes like 30 lines of code to have a cross platform ini parser.
I can definitely see the need to support human readable hierarchies in some cases, but it seems to happen way too often when it is unneeded.