Picard should allow users to specify which tags they want to write to their files instead of just having generic "Clear existing tags" and "save tags to files" options.
Ideally for each tag you could choose whether to
1) Use MB data and leave unchanged if MB has no data
2) Use MB data and delete if MB has no data
3) Leave unchanged
This ticket overrides (at least) http://bugs.musicbrainz.org/ticket/704 and http://bugs.musicbrainz.org/ticket/942.