I think that's just contemporary songwriters, Tom, that want everything to make sense (I have yet to write a "tra-la-la" chorus, pray god I never will). My guess is that in the old days, with fewer instruments available, nonsense words might have played the part that an instrumental bridge would today. And sounds would be chosen for effect as much as anything - "ling" has a kind of ring to it, for example.

Neither is the art of nonsense words in song forgotten today; take scat-singing for example. Now, we know for a fact there was no ancient language behind that one, as it was invented in our times, so to speak.