Important: Don't forget the functionality of the playlist: keep a list of things to play!. Don't confuse it with what you want it to do (keep track of what's in a picture)!!
You are talking about a huge paradigm shift from where "the file contains the information" to one where "where the file IS contains the information".
Let's follow your logic: Firstly, kill all fields that MJ can or ever hopes to store. Secondly make a thousand and one playlists under different groups for Genre, Artist, BPM, Colour of the Underwear that the Recording Technician was Wearing, etc. Oh, and of course, your regular playlists.
You'd be negating the need for Media Library because we, as the music collection owners, would have done all that hard work ourselves...
Essentially you're consiering moving to a completely different kettle of fish here -- a relational DB. MJ has some of the functionality of a relational DB, but only in the playlists.
There is a lot of merit to this approach and it means that files only contain fields that are relevant to them. However, it is a HUGE change in mentality from where MJ is now.
Let's get back to what we have now:
Having "mum, cat, dog, bird" in a photo field is fine. If I enter a search for people=cat,bird,dog won't MJ find the photo concerned??? There is a difference, is there not, between people="cat","bird","dog" and people=[cat],[bird],[dog].
Along this line, however, you really need to make a more intelligent wizard that can handle, for text fields, things like "includes", "starts with", "ends with" and "is".
Once this is done, I think that your suggestion is nullified because it is then easy to create lists of photos with "me and mum" and it is easy to create a list of photos with "me and not a dog" (NO pun intended!!!).
At the end: I don't think that playlists have a function being stored in a file. If I send a picture to friends over the ocean (even you, for instance!) it is more "useful" when you browse the image in a different program (eg: Windows XP -- actually, does that read image metadata??), you don't want to know what playlists a song is in, because not all playlists are people.
I may have a playlist called "relax" which has some slow tunes and some other soothing images. You, as the recipient of my piture, don't need to know that - and all that information has bloated the size of the picture. If there is a field telling you that the people in the picture are, Bob, George and Mary -- that is more useful to you.
Isn't it?
I hope I'm on the right page as you!!