I've always found this to be a pretty silly topic, but being a Dylan fan from way back, it's hard to stay out of the fray.

I think one thing that is lost in all this is that there are a gazillion songs out there, and when you narrow it down to songs that are in the style(s) Dylan does (American/British folk-based, in a comfortable singing range, constructed out of simple melodies over a few rudimentary chords, etc.), it's almost impossible to invent new melodies that aren't remeniscent of some existing melody or other. And many of the examples cited in this thread aren't exact copies of existing melodies, but are merely similar. Given how prolific a writer Dylan is, and the inherent limitations in his song styles, it's difficult to imagine what he might have done otherwise.

This also comes up in American country music, rock and roll, and especially blues, where the liberal borrowing of both melodies and lyrics is almost a defining element of the style. And along with various folk songs and stylings based around the Smith anthology and other similar sources, those are the three main elements that come into play in Dylan's music. But I rarely see these sorts of discussions about other songwriters, or even other folk-based performers (unless they're cited by people defending Dylan against these sorts of charges). Think about how many knockoffs there have been of Chuck Berry's songs, and ask yourself how many people have been taken to task for that (other than Brian Wilson for the "Sweet Little Sixteen/Surfin USA" connection). It makes one wonder whether those who object the loudest have other gripes with Dylan, and this is just an area where they think he's vulnerable.

In the end, I don't care if Blowin' In the Wind sounds kind of like No More Auction Block -- I probably would have never heard the latter if not for the former, and frankly (in my opinion, of course), Dylan's song is the better of the two.