Nobody but Picard/mutagen can read the tags that are written. It seems counterproductive to save files that nobody can read outside of Picard/mutagen.

(if I remember correctly)

Last I checked, popular programs like iTunes, foobar2000, and Mp3tag couldn't read Picard/mutagen's multi-values. Since I last checked, however, Mp3tag at least has supposedly rewritten code, so I don't know where compatibility currently stands.