Fiddle tunes have been at the heart of traditional dulcimer repertoire from the beginning, along with other common music of each era. I suspect also that the term "fiddle tune" is sometimes used in error. Some of these tunes were probably first played on other instruments. Certainly anything composed by Turlough O'Carolan was composed on his harp. Here's a couple other points to ponder:
In very early times, the chord-based harmonies that are so standard in modern music were unknown. Fiddlers,harpists, and players of diatonic concertinas played drone backup or else no backup at all. When other instruments played accompaniment it was not based on chords, but harmonizing notes. This may have been parallel 3rds or 5ths, or possibly 2nds, 4ths or 6ths. By definition, a chord contains at least 3 notes. These 2 note backup parts are 'intervals', not chords.These drones above are all scale tones taken from the scale named by the root note. All, to greater or lesser degree, will harmonize with other notes from the same scale. But not all harmonize equally well with each other. The 5th is the one that harmonizes best with the greatest number of the notes in the scale. It works with any mode of that key. The ancient ones may not have understood music theory as it is taught today, or even as taught in Mozart's time, but they recognized what sounded good. I suspect they tried different combination's until found what pleased their ears.
These drones transferred from fiddles and harps to dulcimers and other stringed instruments as they were integrated into early ensembles.
The interval of a 5th above the root note D is A. This is conspicuously identical to the common I-V dulcimer drone for the key of D. And this is the common drone a fiddler would use, and still do in real Old Time fiddle playing. Bluegrass harmony, and fiddle playing, is much more chord based. Orchestra music always was composed and played by highly trained people, who understood chord harmonies. Folk musicians, especially in eras where most common people had no formal education, were based on more ancient systems. Modes instead of scales, etc. The Old Time banjo and the mountain dulcimer are throwbacks to these earlier times, and have retained much that has otherwise evolved away from drones. This is why, as a player of clawhammer banjo, we use multiple tunings, similar to dulcimer players. The tuning contains the mood the song requires to sound as it did in the early days.
Don't take this to suggest all players must embrace fiddle tunes. This is just a history lesson. You still have to play the songs that put a smile on your face. If you play fiddle tunes, with or without drones is purely up to you. Go and enjoy.
Paul