When Willie Nelson sings, he's so far in the pocket it sounds like he might be singing a different song. His guitar has a second hole, a worn out spot just down and to the right of the main one on his acoustic, and it sounds like he hasn't changed the strings, maybe ever.
And "singing" is sort of a strong word, anyway, when really, as Katie says, it sounds like he's doing a very loose spoken word interpretation of whatever song his band happens to be playing.
But when it all comes together, you realize he's doing exactly what he wants to do - not because he can't play in strict rhythm, or right on pitch, or with a new instrument - but because he wants to do it exactly the way he's doing it.