Lots of singers can't sing
besides Neil Young
Aside from Bob Dylan, already mentioned, there is Tom Petty, Ian Anderson (of Jethro Tull), Janis Joplin, Jim Morrison, Ozzy Osbourne (although he is understandable singing, not talking), Ric Ocasec (of the Cars), Stevie Nicks, Deborah Harry, Ray Davies, Roger Waters (of Pink Floyd)..
The list could go on and on. All of the above are people in my collection that despite their less than stellar voices, are singers that I enjoy.
In fact, I think there are many "bad" singers that I would rather listen to than some of the polished "good" singers.