I agree that there are many advantages to building up pre-conceived notions on many subjects...we are simply bombarded by information in this age, and building up cognitive shortcuts or "schemas" can help us process information better and make smarter decisions. See Malcolm Gladwell's excellent "Blink" or Gerd Gigerenzer's even better "Simple Heuristics That Make Us Smart" for a deeper dive into how we process information and make decisions based on intuition and experience.

The problem with "Preconceived Ideas" (I won't use the word prejudice, because of what it often implies) is that these cognitive shortcuts get us into trouble way too often. There are countless books out their on Cognitive Biases that clearly spell out the decision traps that we get into because of these shortcuts. If we are not aware of these heuristics and biases, these shortcuts, these "Preconceived Ideas", these prejudices can get us into trouble and we often end up making quick decisions that are quite often WRONG. I'd suggest looking into the works of Thomas Gilovich, Amos Tversky and Daniel Kahneman.

We make a lot of decisions every single day, and very often we make bad decisions. Too often the difference between a good decision and a bad decision is based not on the level of intelligence of the decision-maker, but rather on the process used. Relying on intuition and preconceived ideas can be a mistake, especially when there is data available that runs counter to our experience and that contradicts our prejudices.

I'm not going to mention the political tilt to this book, because I feel its irrelevant. Yes, I believe that conservatives are more likely to fall prey to cognitive biases and to rely too often on prejudices, but there are overwhelming examples that show that the ability to take "shortcuts" is not limited to one side of the political spectrum. This is a plea: recognize when Preconceived Ideas help your decision process, and when it hinders it, and you'll become a better decision-maker.

Much of the criticism directed here against this witty and insightful short work laments that it elaborates the obvious. Such critics of it are, of course, to be commended for their own prior level-headedness, while at the same time they should realize they're overlooking Dalrymple's argument that large sections of the media and academe have in our day genuinely lost sight of the obvious. And the obvious, as the aphorism has it, is the hardest thing to point out to people who've genuinely lost sight of it. Even Socrates recognized that people more often need to be reminded than, in the fashion of the Sophists, newly instructed. I add C.S.Lewis' sobering insight that even God didn't bother to be original.
Some of the hostility toward this book seems motivated, as well, by a simplistic religious fundamentalism, since it's true Dalrymple declares himself a non-believer. But he who is not against you is with you, as the Bible asserts. By all means read Burke and Kierkegaard, but I suspect each of these old worthies would have seen the basic thrust in Dalrymple, as in Plato, is in the direction of the Good, the True, and the Beautiful.