It's my fave bit of the song! Purely because it's just so flippin good to sing along to.
I think you're right that he never intended to leave in the place holder "lyrics" (i saw him talking about it in a doco)..... But i reckon (and i seem to remember he also reckons) that they are part of what makes the song. [edit for the avoidance of doubt: I'm still talking about Paul Simon here]
Anyway, I'm sure there are loads of examples, from virtually every genre, of "good" and "bad" non-verbal lyrics, and for every example you could find plenty of people arguing on both sides of the fence.
For my part, i agree with Boydie that, used right, they can help people connect emotionally. And a big part of that is the singalong factor. Who
hasn't drunkenly belted out a rendition of "wo-oah livin on a prayer".....and felt all the better for it too!
I posted that after hearing Sting doing a live version of Roxanne where there's a 2 minute break in the song while he just goes ROXXXANNNNE-OH WOAH OH OH ROXXXXANNNNE OH DE DOOBY DOOBY DOO". And I wanted to strangle him.
I can imagine that would have been blummin annoyin!