I agree these are under-used but they face the same issues as the other methods like putting the data in text near the image on a web page or presentation slide. These fields, same as all Exif metadata, can be overwritten or removed by anyone with access to the file. The data can be faked by someone intending to deceive, or it could disappear when posted to big sites like Facebook or Twitter who routinely remove Exif data by default (presumably to protect the majority who don't understand how GPS tagging works on mobile phone photos, etc).

Once the photo metadata is gone, it is too easy for others to claim it is an 'orphan work' and avoid liability under copyright law. At the opposite end of the spectrum, people like me who release most images as CC0 are annoyed that that license tag was stripped from the metadata, preventing others from freely reusing them. I use and rely on Exif tags a lot but they are fragile and you cannot rely on them staying embedded with your images once they hit the web.