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.
.XIB files are XML archives of the objects (and their properties) required to build an interface and that's worked quite well for nextStep and OSX/iOS :-)
