ITPC is not all that new either and also have a few obsolete elements kept just of compatibility reasons.
Here to some intresting:
The number of EXIF-elements in my testfile, according to EXIF Tools, are no less than 266!
In EXIF Data Viewer i see more modest 58 supported elements that I can edit and in PhotoLibrary of Photolab there are just three open elements to my knowledge beside the elements it just displays. The rest are ghosted!
Another thing I observed is that EXIF Tools just can´t handle our scandinavian characters like ÅÄÖ and åäö at all. Quite a few other countries like Germany hos som odd characters too. That might also be a problem if working with EXIF Tools (who might work with ASCII or something. EXIF Data Viewer handle these characters perfectly fine. When I was a productmanager för Microsoft products and Windows-software a few years in the nineties at the biggest distributor in the Nordic Area of Europe. A lot of my work was to help our 200 different software vendors to adapt their softwares to the Swedish market. As an example a Windows software for emulation an IBM AS/400 could have one character set downstream (EBCDIC), an other in Windows clipboard (ANSI) and a third when typing (like ASCII). It took a few years to get the americans to understand. That´s also a reason why FotoWare doesn´t recommend to mix MAC and PC-versions of their client software Fotostation in the same company network.
Well the americans used to ask me if we really needed those strange character (like ÅÄÖ and åäö and I used to turn the mailing back excluding some of their characters asking them is they understood what I wrote. We used to say that ÅÄÖ was the biggest employer in the IT-industry in that time. Microsoft also sold Microsoft Works to small associations and their customers directly complained about they couldn´t print the paragraf signs they could see on screen, but not print before somebody (like I) rewrote the printerdrivers for them. So characters matters too.
I think we might have a chat with Kirk Baker or some other developer at Camera Bits in order to ask them to fork elements like IPTC Title/Headline, Subject/Description, Rating and Comment just to mention some of the most important. Both Lightroom and PM Plus 6 also use the Color Class-element to color tag the files in IPTC. In PM Plus we can even selecy a color schema that matches Lightroom to make metadata exchane smooth, but in PhotoLibrary that element is ghosted too. They just use rating. If Camera Bits forked the data or at least opened the EXIF namespace in XMP we could easily do it ourselves with the excellent variable-support there is in PM Plus.
So my conclusion is that it´s really a lot that can go wrong when pushing XMP-metadata both ways between any of the programs I have talked about like Photolab 5, Lightroom, Photo Mechanic and not the least with the very importand metadata viewers in Windows Image Viewer or Windows Explorer Properties Info Tab. I think it´s really important that our metadata displays correctly not just in our IPTC-centered software bot also in EXIF of compatibility reasons and I just can´t understand why this isn´t fixed a long time ago - hello Microsoft and all metadata-software manufacturers (no named (except Microsoft )and forgotten), time for some action before you fall back on the pillow!