There's a Scots Gaelic song tradition called puirt-a-beal (mouth music) - this is primarily music for dancing to (though I've heard a theory that it originated on the islands as a way to while away the long working hours). As a result, the lyrics to such songs are only secondary; it's the RHYTHM that's the most important feature, and the lyrics are generally simplistic or nonsensical.

It seems to me that the nonsense syllables in modern traditional song serve the same kind of rhythmic purpose, asserting the music's connection with the dance tradition.

And sorry, I have to be a big music geek and say that solfege (using do-re-mi-etc.) is really very different, and not nonsense syllables at all. Each solfege syllable stands for a specific pitch in the given key, and thus has a specific theoretical relationship with every other syllable. Okay, the syllables themselves are arbitrary, but they DO stand for something.

LD