I occasionally use software that isn't as mwg compliant as iMatch with as result that the value of some IPTC tags is different from the XMP counterpart.
I would be able to populate a category with those images if I could compare two tags with the @MetadataTag() function (if the value is different they are added to that category).
I think this could be also to your benefit if you had a category with all the files where the metadata isn't consistent.