One of the recurring themes in what Penn Jillette says, that I am often impressed with, is his view that "I Don't Know" shouldn't be frowned upon. "I don't know" is not a swear. It means you are open to new ideas. But at the same time, I think that simply saying "I don't know" to a question is a failed thought.
In the 18th Century, when lightning struck a building in a town (often a church since they tended to be the tallest), people would ask why that happened. The common answer at the time was that it was God's wrath on unfaithful church followers, or those who had sinned. They didn't say "I don't know". They "knew" it was God, and therefore no further investigation was required.
A deist, as well as brilliant inventor and statesmen, named Benjamin Franklin instead answered the question "Why did lightning strike that building" with "I don't know". But had he stopped there, and simply said "I don't know", he would have been no better than those who had said it was God's will.
The beauty of "I don't know" is that it conjures in many people the desire to follow it with "But I want to find out". In fact, if humans didn't have the desire to find out what they didn't understand, we'd still be no better than cave men today. Franklin created experiments and discovered many of the properties of lightning as a result of his desire, and created one of the most useful inventions of that period; the lightning rod.
At the time, the lightning rod was crucial to the development of society as we know it. When lightning used to strike buildings, it would not only cause mass devastation on that building, but it would start fires that would potentially rip through a town. The lightning rod not only saved many towns, but also allowed us to create taller and taller buildings, knowing that the force of lighting could be harnessed and safely controlled.
Given the importance of that invention, Franklin could have made a large sum of money with it. Yet Franklin chose not to patent his great invention, because he thought it was too important to society. The morality of that decision is difficult to question.
And so we must remember that "I don't know" is never a good enough answer to a question. Yes, it is important to remain open to new ideas, and allow yourself to be proven wrong. But the real power in "I don't know" is that it allows you to go out and discover the right answer, and not wait until someone else comes along with a better answer.