OK, maybe not usually, especially for classical music, but it's more than likely that you'll sooner or later run into a track (or album) with multiple composers.
Generally, the tags that are used by most people are title, artist, composer, albumartist and genre (the bare minimum being just title, artist, albumartist). Of those tags all but title and albumartist are likely to be multi-value. Even the ID3v2.3 spec, which is far from perfect, anticipated that and allowed TPE1 (artist), TCOM (composer), TEXT (lyricist), TOLY (original lyricist), TOPE (original artist), to be multi-value (not genre, though).
When dealing with FLAC, those multi-values are usually written as multiple tags, and they only show in column view if you use $meta_sep(), which Mp3tag does in artist and genre column by default. So why not just add it to another one of those most-used tags – composer?