Felix I remember I was alredy giving extensive explanation about this, but probably that was on different forum. The main thing is that XMP format for people is very poor and is not adequate for puproses of people data synchronization. It lacks such basic thing as peson ID to say the least. That is why ini is the primary place to store this data.
Yes, making copy of this data to XMP might also be useful sometimes, but has a lot of disadvantages too. Just imagine you rename a person, and then program will have to update 10 000 images to keep data in sync. And what if for some reason it failed to do so (you closed program before it finished)? Part of files will have different names, that is another huge challenge for conflict resoltion algorithm.