It's not just the vocal range, Hawk. I don't think Dylan or anyone with his vocal limitations (probably not a lot more than an octave range, just to name one aspect) could even begin to imitate or sing in the style of someone with a clear, resonant voice.

This is not to say that Dylan can't carry a tune -- he obviously can -- or that his stylings aren't expressive or entertaining or poignant. It's just that, IMO, there's a difference between being a great singer and a great a) entertainer, b) lyric interpreter (especially of one's own poetry), c) instrumentalist, d) provocateur, etc.   

Several people on Rolling Stones list are people I'd love to see and hear in concert and whose music I also have great appreciation for but whom I don't consider terribly good vocalists - certainly not among the "100 best singers of all time."
Amos mentioned Jimmy Durante. Another example, IMO, of a wonderful enertainer but hardly a "great singer."

I guess part of what bothers me about a list like this is that there are plenty of wonderful singers who neither write songs nor play instruments. If they are to be 'bumped off' lists of "best singers" by other entertainers because the latter can write great songs or are instrumental vocalists, then where is the acknowledgment for being just that: an excellent SINGER?