If my above reply to @Yvanz is right, then I think we do not need to set a “type” at all! Not for songs and also not for instrumentals! Why not?
Because in this option, the Music Brainz is not an Encyclopedia of compositions, but an Encyclopedia of artists performing certain compositions! To explain: you look up an artist and see which works/compositions the artist has in his/her repertoire, or you look up a work/composition and see which artists has that in their repertoire, and depending on the artist you can guess yourself if it will be an instrumental or a music piece with lyrics, because, if the main artist in question is a vocalist, you can expect lyrics or scat singing or spoken text (rap), while if the main artist is a saxophonist, you can expect a (mainly) instrumental version of the work. Am I right? If so, then @Freso 's statement in the other thread quoting “If you doubt, leave it out” not only is a nice statement, but also is a wise statement, because it does not really matter to set the type, when looking for artists and their “works” (works in the meaning of Music Brainz).
However, I still am in favor of being able to set a type, although I can go along with not setting a type 
If my above reply to @Yvanz is not correct, meaning I misunderstood what he said, then please say so!