Dylan's pretty exclusive use of the flatpick in his early acoustic phase is surely connected with his justification of wanting a band to sing with, as interviewed in the Scorses film. I cant remember the exact words he used, but it was something along the lines of "wanting more power under the song". This is what he would have got from the flatpick, in his solo performer pre 65 phase. He was prepared to play with his fingers if he wanted to be gentle(eg Corinna Corinna on Freewheeling), but broadly speaking he was a savage flatpicker, which is what you need to be if you are going to do songs like Hard Rain. You only have to look at the way he held his body, and face, while singing, to see that a flatpick was a necessity. (Perhaps "necessity" is a bit strong, Leadbelly did it with steel finger picks and a 12-string, but the flatpick is the easier option).
   The integration of the physicality of the performance with his mind was astonishingly dealt with by the Alan Ginsberg description of the column of his breath, a very interesting attempt to put something very inexplicable into words.
    AS a relevant point, contrast Dylan's wearing of boots with Baez's occasional barefeet. You need some weight on your feet to develop a style like Dylan's. I ahve always found it difficult to play good backing guitar when wearing shoes, I nearly always wear boots for flatpicking, though I can fingerpick in carpet slippers at home.