Should Authors Use Um And Uh In Their Dialogues

Now, uh, where was I? Oh yes! Now I remember.
Everyone at time stumbles with the words that come out of their mouths in real life. So why shouldn’t your characters in story do the same? Of course some characters are more eloquent than others, but if you ignore the simple things that people do from time to time, you simply cannot expect to write convincing, natural dialogue that readers will believe. If everyone in your story speaks perfect “English” then, whether you believe it or not, your readers are going to notice and perhaps not connect very well with your characters.
If someone stumbles during a scene, looking for what to say next, then there is nothing wrong with using a timely, “um” or “uh” to depict that if the character actually says “um” or “uh” at that point in time. If the character just pauses then of course you use “…” or something similar. But sometimes a pause is more than just silence.
Now, overdoing the “uh”s and “um”s can get a quite annoying. If every other sentence out of every character’s mouth contains multiples of one of these filler words that is going to break your flow and probably annoy the reader. Sure people use these placeholders in real dialogues to buy time to find their next words. But all except the most stammering of people don’t fill entire conversations with them.
Perhaps you have one character in particular that really has a hard time communicating and uses these placeholders in conversations frequently. That is a very good use for them so long as they are kept down from an annoying level and they certainly should not be used by every speaker frequently if this were the case.
It seems to me that if you have a conversation going between two people in your story, either an “um” or an “uh” should not appear more frequently than every three or four sentences a particular character speaks and only if he or she is the only one using such non-words. Of course a notable exception would be if the situation were to really call for frequent use of these fillers. Such as, for example, one character catching another doing something untoward and the later is really caught without much to say. Then you might be able to get away with a whole conversation (on that particular character’s side) filled with these words.
So I say use them. But use them sparingly, unless the situation really calls for it, and perhaps as part only of a particular character’s speech pattern only.