The main purpose of a radiused fretboard is to facilitate barring across all the strings with a single finger. Aaron O'Rourke uses his pinky for that purpose. Stephen Seifert uses his ring finger. Barring with a single finger instead of using three fingers allows more fingers to fret strings above the barre, enabling the playing of more notes while not losing chord tones. To my knowledge, the first luthier to build a dulcimer with a radiused fretboard was David Beede when he designed a dulcimer customized to Aaron's specifications.
A second reason a radiused fretboard might be useful is in playing single-note runs on the middle string. It is easy enough to angle a flatpick upwards to play single notes only on the melody string or downwards to do so on the bass string without hitting the other strings. But the middle string is more challenging. A radiused fretboard essentially raises that middle string up a bit relative to the melody and bass strings, making it easier to play by itself.
I have not opted for a radiused fretboard because I rarely barre with a single finger and also because I'm afraid that I would get used to it and never want to play all my dulcimers with flat fretboards. But this discussion would benefit from comments by someone who uses a radiused fretboard rather than all of us who don't.
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Dusty T., Northern California
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As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie
