It would be fine to see values coming from MusicBrainz in the Metadata Pane.
If you load files into Picard and get data from MusicBrainz you have both, the tag values from the local files and the values from MusicBrainz. So far so good.
If you have e.g. tag values in the local files, originally coming from a relationship from MusicBrainz, and this relationship was deleted (for what reason ever, e.g. if it was wrong), you get no information about this.
To get rid of such wrong data you have to know it and do it manually or you must set option “Clear existing tags”. First option is I’MHO not manageable in general. The second option has disadvantages if you would also keep other tags, not coming from MB.
It would be fine to see values coming from MB in the Metadata Pane, e.g. by having a different (background) color or with a new column that indicates the source for the New Value.