This feature request is only directed toward PMP, because it would be hard to implement with the regular PM.
When variables or IPTC templates as described in this FR are changing, their occurrences are updated with their new value. For this, PMP would store the metadata for each picture in it's database, but not replacing variable tokens with their values there. The replacement would only happen in the actual image or sidecar file.
For example, I use the variable for location taken in the description field. The location taken field, when filled in by the reverse geocoding feature of PM, sometimes contains the house number of a building, when I want it's name. I currently have to change this in the location taken field, as well as in the caption. Of course, search and replace can help me with that.
It would be more convenient though, if I only needed to change the location taken field, and the caption being automatically updated by PMP.