Paul McCartney is, with little doubt, one of the most talented musicians alive today. Music flows through his blood. Put a musical instrument in his hands, and it instantly becomes a part of him, ready to bend at his will to whatever enters Paul's mind.
However, he is completely unable to read or write a note of sheet music.
He just never learned. Why would he need to?
Funnily enough, McCartney wrote a symphony a few years back-- it was called Standing Stone-- but because he could not write or understand sheet music, he had to compose the symphony with a computer program: He literally had a system rigged up where he could compose by playing into a keyboard set up with a computer that would transcribe the music he played into sheet music, then let him tweak it.
Still, just think: if someone so good can get by wholly without knowing to read or write sheet music, then that gives so much hope for the rest of us.. because it means all you have to do is excel at what you want to do, and the conventionally "necessary" skills aren't..