There are many composers that were famous for their operettas (Jacques Offenbach, Johann Strauss, Jr. etc.).

For example operetta "Die Fledermaus":
http://musicbrainz.org/work/88504b20-f1e2-4316-ad45-347730af6b5f

MB currently does not have direct support for Operetta work type. And Opera does not seem the right fit for those.
Since there are hundreds of operettas and it was popular in 1930ties to release LPs with songs from operettas as easy listening music,
it would be nice to have direct support for this type.

I was not able to found any previous suggestions to add this type (except for one proposal to significantly extend work types).