If you're editing, and you hear an "um", then, of course, you'd want to take it out. But one of the things that can ruin the edit, is if the sentence still needs it's "break". What I mean is that normally when people say "um", it means that they're thinking of what next to say, so they just blurt out "um". But when you take out that space, the listeners can hear/tell that something was cut out. Think about how robots would speak; they just say the words with no expression and no break until they finish their sentence. That's how it can sound if you don't clean out the "um" right. What's the secret? Replacing it with a nice (not totally silent) piece of sound that has nothing except the sound of the quiet white noise in the background of the person who's talking. That way, it sounds like the speaker is thinking silently, instead of the annoying "um".