I do a lot of talking in front of clients, running design meetings etc... At my last place we had a jar for folks who had client contact. Whenever they started a sentence off with "ummmm" they had to throw a quarter in. Long ago, I learned how not to do that. I am nervous in front of folks, like everyone is. But there are things you can do and it is public speaking 101 to not do that. obviously huge pauses are bad, but things like repeating or rewording the question is a huge gap filler. There are other things that can give you a few second to get you mind around a question.
In this case he looks stupid and really shows how bad of a speaker he is without a tele prompter. The questions were canned, notice how he called specific reporters for questions. He knew what they were going to be, and had been preparing since Friday for them. That was just a very poor display of facilitating a press conference. As bad a public speaker I think Bush was, Obama should really limit his adhoc conferences, because he is worse.
As far as you saying ummmm when you speak i don't care, you're not the President of the US. he is the talking head for us, and makes us look ignorant or worse stupid when he pulls the kind of crap he did Monday.