This article discusses "disfluencies and discourse particles" (aka "uh and um") and posits the interesting notion that no one mentioned them as a problem until phonographs were invented and could record speech for playback later. That is, in "real time" we generally don't notice the little space-fillers that give the speaker a bit of a pause.
They mention the doubtlessly invaluable organization Toastmasters "fining" their speech-making members a nickel for each um. That actually solved a bit of a mystery for me. Hands-down, the greatest lecturer I've ever heard (and I went to a university with scads of great lecturers) was the writing seminar leader Robert McKee, who combines a hipster's insouciance with a cinephile's obsessive command of his material and a storyteller's sense of suspense. I've been to a couple of his all-weekend seminars, and really, he's so good that IN HOLLYWOOD, all these petulant Colin Farrell lookalikes and dead-eyed zombie gorgeous people TURN OFF THEIR CELL PHONES AT HIS COMMAND, then sit there in a college classroom for 20 hours or so listening to him and assiduously taking notes and then, at the end of the weekend, rise spontaneously to give him standing ovations. This guy really is great, and I say this as someone who has given many, many writing workshops myself. I wanna be him when I grow up, only I think we're both running out of time.
Anyway, I was talking to another writer after we saw McKee in another venue (at another organization's conference). He was typically awesome, mingling casual anecdotes about his golf buddy Paul Newman ("Yes, ladies, he really was what the rest of us guys call 'unfairly handsome'") with tossed-off brilliant analysis of scene design (one I always use now: "The end moment in a turning point scene should reflect the type of story it is, so the first scene in a romantic comedy should have a romantic comic ending event"-- sounds simple? Well, try it if you've ever been told that your romantic comedy just doesn't feel like a romantic comedy, try making sure that the very last bit of the first scene is romantic and comic). So when I marvelled at the insightfulness, this writer said bitterly, and, to my mind, obscurely and oddly, "Can we say 'um' and 'uh'? I wish I had a nickel for every time he said one of those!" Later she told me she'd been president of her local Toastmasters chapter. Anyway, this article mentions that Toastmasters used to fine members -- you got it-- a nickel for every "disfluency". (Way to miss the point, huh? :)
Anyway, linguist John McWhorter points out that these show up in most every language, and the "tone" determines whether "un-uh" and "uh-huh" are negatives or positives. And they're mostly vocalisms (though there are some equivalents in written language, n'est-ce pas?). And they are actually more "fluencies" (easing discourse) than "dis"fluencies. But interesting anyway, and if Robert McKee does it, that means it's cool. :)