When I write dialog, I include a lot of "uh", "um" and such. It seems natural, especially when explaining something at length without preparation. It is definitely realistic.
This morning, I watched Buckaroo Banzai. I don't know if there was a budget problem but there is a lot of awkward movement, people taking a few steps then returning to where they started, then traveling in a new direction. I think the technical term is 'hitting their marks' or failing to. I found it pretty distracting.
I have read snide reports of how often characters overuse a line or phrase. I would hate for someone to count the "um"s in my current draft.
The words have to be real, reasonable for the person. The characters don't have to sound like idiots, unable to complete a coherent thought. If someone is unsure, lacking confidence, "uh", is a good shorthand for that. There are reasons to use the word. Just not very many.