I add my voice to the praise of literacy.
To deny, in all cases utterly without exception, the importance of written sources to the transmission of songs in a literate civilization is as silly as insisting that a given written version of a song is the only true version.
I strongly suspect that some collectors' informants learned some of their songs from books or broadsides, or from people who learned them from written sources. I don't recall any know case of "John Smith on a hill in Kentucky" being ashamed of having a copy of The Scots Musical Museum, or any other book, in his house. I think it would be a low-risk generalization to state that oral and written tradition are engaged in constant give-and-take. Song-collecting is one sub-process of this give-and-take.
T.