I think Dylan's voice perfectly suit his songs; the way he sings Sara is one of the most gutwrenching things I've ever heard.
I don't think there's such a thing as "technically" good singing, or "technically" bad singing. Sara in the hands of somebody else may well just be hideous. The same reason I think Tom Waits is a brilliant singer.
The same thing applies to filmmaking, over which we've had a debate before; if something is "technically good", then it must move you in some way in order for you to describe it so.