Excellent point made, Frank, as you so often do. I really like Townes Van Zandt's voice, for example, when singing his own songs. In fact, I like his singing much better than his voice, if you follow me.
Along this line, Steve Earl has a voice that rubs lots of people the wrong way, but few sing with more conviction. The early Dylan had the same kind of voice, as does your friend Mr McGuinn, although I believe Roger's tonal quality was very distinctive. One of my favorite singer/songwriters is Jay Farrar, who fronted Uncle Tupelo and Son Volt, and his voice has a mournful nasal tone that some people find annoying. To me it has that Appalachian Mountain quality that lends authenticity to the words.
I do think Garrison's voice is very like his personality, and he can be repulsively down-home and folksy at points when you aint in the mood, but for those of us who enjoy rhubarb pie, it often hits the spot.