If depends on your definition of folk songs of course, but if the source of the idiom is particular to a cultural group, especially a disenfranchised one, good taste dictates one should not mimic the speech patterns, or perceived speech patterns of that group.

I accept there are no easy answers but I've always struggled with The English Folk Voice as a universal aural meme and 'negro spirituals' by posh white folks is an extreme version of that dilemma. Some folk forms translate more readily than others into the lingua franca and their mutability or resistance depends on a variety of conditions.