I remember that he had an operation for vocal chord polyps around after Quad.
It's because he never learned to sing right, or wasn't coached.
When I saw the Freddie Mercury Tribute in England he and Plant were my major disappointments.
I think they have not used their voices correctly, and, while Plant has done a masterful job molding his voice into a sweet bluegrass croon, Roger insists in singing like it was 1971