First of all, numerous music artworks are composed, performed and copyrighted by multiple artists (e.g. pop songs, musicals, movie soundtracks, and stage play recordings) and can be categorized into multiple genres (e.g. classic rock and hard rock). It would be unnatural and not parsable if multiple tag values are squeezed into one tag field, not mentioning the artificial separation characters to separate these values.
Secondly, unlike the highly structured ID3 tags for MP3 music files (where there seems no standardized method to implement this feature), the Vorbis-comment tags that FLAC files use are fairly flexible in that Vorbis comment tags allow arbitrary tag names and any number of entries of the same tag. This flexibility leads to the most natural and straightforward solution for implementing this feature by just adding multiple entries of the same tag (the key) but assigned with different value for each key. Since Volumio’s primary focus is to play and organize audiophile music, FLAC is probably the most common format used by Volumio’s users and the easiest format for this feature to be implemented (the DSD/DSF format uses ID3v2 tags and therefore the implementation of this feature needs some best-practice design considerations).
Thirdly, popular music tagging software programs, which can process FLAC format, are able to handle the feature, including MusicBrainz Picard, Kid3, and EasyTag. Some music players, ncmpcpp (a front end of MPD) and Flacbox (a FLAC music player in iOS), can process and display, say all multiple artists of FLAC files, correctly.
Lastly, take the song Do-Re-Mi from the movie The Sound of Music as an example:
ARTIST: Julie Andrews, Charmian Carr, Heather Menzies, Nicholas Hammond, Duane Chase, Angela Cartwright, Debbie Turner, Kym Karath
ALBUMARTIST: Richard Rodgers, Oscar Hammerstein
ALBUM: The Sound of Music (50th Anniversary Edition)
COMPOSER: Richard Rodgers, Oscar Hammerstein
PERFORMER: Julie Andrews
COPYRIGHT: Rodgers & Hammerstein Organization Production