Based on http://tickets.musicbrainz.org/browse/STYLE-457
Right now the musical theatre guidelines especifically say "the release group and release artist credit should be to the composer(s), rather than performer(s), lyricist(s), or librettist(s)" - before going on to list five famous exceptions where the lyricist/librettist should indeed be added to the AC anyway.
Classical and soundtrack releases don't specifically block lyricists or librettists, but they do specifically say to use composers.
I can see no good reason to block credits to lyricists and librettists if they're who the release actually credits. This seems relatively common for musicals. While outside of musicals, librettists and lyricists being given credit for a release is less common, I've definitely seen classical releases with, say, songs on Verlaine or Shakespeare poems exclusively, or operas where the librettist also gets a prominent cover credit.
Is there any good reason not to allow adding those as artist credits?