Recently I have been involved in a discussion about whether voice teachers should teach sight-reading. Several people got on high horses and said that of course we should develop “all facets” of being a singer. I say “bosh”. Reading music is peculiar to the Western classical and industrial tradition and is not a part of the musical life of most musical cultures historically. Indeed, many successful modern musicians here in 21st century America are not music-readers. It’s just not necessary for everybody.
People who don’t read almost always have sharper ears, learn fast by ear, and develop a feel for phrasing quicker than readers. Nowadays there are so many ways to learn a song. Long gone are the days when sheet music was the best way to convey a piece of music over large distances.
But to the main point, when working with a singer on cultivating their voice, there is so much to do that working on reading would be a poor use of expensive voice lesson tuition. I am charged with getting them to sing healthily and well, which is completely unrelated to transferring notation to performed music. It would be better for them to go to a coach if they want help with interpreting notes.
The main situation where I work with a singer on learning to read music is if they are young, or if they specifically ask for help with it. But I use workbooks and self-study packages and check in on them for maybe five minutes per lesson.
So having said “There is no universal need to sightread,” that leaves the obvious question, “Who does need to be a good reader?” These people: choir members, composers, teachers, classical solo artists (opera, recital, chamber), people who perform with readers, and students in formal degree programs. Most others can get by without it in their real-world musicmaking. Even some of those who theoretically “should” read, don’t.
I’m glad to be a good sight-reader, and I encourage people to learn. But it is not necessary in order to become a good singer.
I don’t like the term “music literacy” since people assume it is an equivalence to language literacy. If you narrowly define it as being able to read musical notation, then it makes a little sense, but there are “readers” who are not adept at making all of the elements of music come alive, and there are amazing performers who understand rhythm, pitch, harmony, and lyrics to a very deep level who don’t read. Is the latter person “musically illiterate”? In terms of reading, perhaps, but in terms of being fluent in the heard language of music, certainly not. And music is ultimately a thing to be heard, not to be read like a book.