Michael Erard explains why:
In the history of oratory and public speaking, the notion that good speaking requires umlessness is actually a fairly recent, and very American, invention. It didn't emerge as a cultural standard until the early 20th century, when the phonograph and radio suddenly held up to speakers' ears all the quirks and warbles that, before then, had flitted by. Another development was the codification of public speaking as an academic subject. Counting "ums" and noting perfect fluency gave teachers something to score.
What's more, "uhs" and "ums" do not necessarily damage a speaker's standing. Recently, a University of Michigan research team turned their attention to phone survey interviewers. They found that the most successful interviewers—the ones who convinced respondents to stay on the line and answer questions—spoke moderately fast and paused occasionally, either silently or with a filler "uh" or "um." "If interviewers made no pauses at all," the lead researcher, Jose Benki, told Science Daily, "they had the lowest success rates getting people to agree to do the survey. We think that's because they sound too scripted."
A previous study explained how "uh" or "um" helps babies learn new words.
The Dish 