Mary, the genre of old ballads referred to were largely either brought to America from England..Ireland..Scotland, or derived from those ballads, back in the 1800s, 1700s and earlier. They were traditionally sung without instrumental accompaniment, and the singer sang in a storytelling manner that often had irregular timing, gapped scales, and other very individualized and archaic characteristics typical of that style of singing.
When guitars were introduced into general use around the turn of the century and popular music began to be produced and more available later via radio and 78s, there was a new commercial and popular market for this 'mountain folks' music. But people wanted it to have chord or guitar accompaniment, and to generally be more 'accessible' to listen to as MUSIC...not simply as a sung story. A Capella sounded to raw and unfinished to most people after 1900. Guitars were generally used to 'fill in the music'. Singer guitar players who were not familiar with a Capella trad ballads almost universally straightened out the timing, unified any odd mode changes, eliminated accidentals and 'in between' notes, and generally made the ballads conform to modern music listener expectations. They made the ballads sound 'prettier' because that's what people wanted...not some odd sounding archaic piece you couldn't tap your foot to.
People like Joan Baez and Jean Ritchie either grew up in the old ballad tradition or else made a study of it...they found the old stylings to be beautiful and compelling, and kept them in their personal interpretations of the ballads, even in their instrumental treatment and timings. Other singers and or musicians embrace the more modern treatment of the old ballads, whether intentionally or by just following their usual approach.
As Rob said, you should do what inspires you personally....there is no 'right' way to play a tune or sing a song...there is just each our own way.
updated by @strumelia: 05/06/16 12:04:14PM
lol
Post pictures!