For mere motals, being able to read music is a valuable skill which can lead to a deeper understanding of the mechanics of music i.e. how it works. If you have the innate musical skills of a Django then the inability to read music wouldn't hinder your progress; however, Django once wrote a serious piece of "classical music", but because it wasn't written down ( Django didn't possess that skill) it has been "lost". I think a "complete" musician should be able to read music And be able to play by ear.