It is good idea but not really killer feature. If you have visual editor it is right format to store UI but defining UI in XML is not easier than defining UI in C. Instead of handling coding errors introduced in C language you are handling syntatical errors in XML... but it is very neat idea nevertheless and I would like to try that for MUI, too.
MUI Royale (Hollywood plugin) tranlates an XML file to a MUI GUI. I think it's easier than within C++ as I never got MUI to work with C++ and with MUI Royale it's a rather easy write down of definitions (execept you need quite some time to read the documentation of all the MUI functions).
