Russ,
That is one quite likely explanation, particularly where the refrain interrupts couplets or where a lot of repetition occurs. It has been shown by Albert Lord and suggested by David Buchan among others that prior to literate society and widespread printing, singers did not all memorise set texts but had the story or theme of the song, and a whole stock of commonplace phrases, stanzas, expressions etc and they recomposed the song every time they sang it. I have tried this and it works. For those not as adept at this the use of a repeated refrain and repeated lines would have given time to compose the next verse.

However by far the greatest reason for nonsense refrains in more recent times (last 400 years) is they're easy to do and an accepted practice in almost all types of music. Much easier to throw in a nonsense line than have to compose something that makes sense.