doesn't personinimage (no matter if in plain iptc or in the xmp version) refer to the name of the person only? (I only read the spec now after your post, so i don't really know for sure.)
if so, my family couldn't use it to tag photos: my father and i, we have exactly the same name. if you tag photos with this name, you can't search for me and only me.
also, in the N900 use case, we have to expect that some people enter contacts as "dad", "home", "sweetheart" and "peter's office". while the name is useless in such a situation, they may still have a unique mail address or other unique properties for these contacts.
are there examples of how to use PersonInImage with information other than the persons name anywhere? sounds interesting if it's as flexible as you say in XMP.