Why not? When is democracy not a "good thing"?
"Tyranny of the majority," as someone else pointed out, isn't a good thing. Pretend for a minute the US was really a democracy (it isn't), do you think we'd have made the strides we have in Civil Rights? After the Civil War, blacks couldn't even vote, so, why would the government have even bothered acknowledging their equality?
Beyond that, to coin an old cliche, what is popular isn't always right. I know it's bad form to trot out Hitler, but it's true. He was elected into office. Another example I've heard is back in Athens where the majority of citizens voted to put Socrates (or Aristotle, I don't remember which) to death because he doubted the gods.
In a small group of people, democracy works fine. As your group gets larger, it gets harder and harder to get things done. There're 300 million people in the US, can you imagine the majority of our voters agreeing on anything? Heck, look at Congress-there're only about 550 (I think 535, but I'm too lazy to look it up. Suffice it to say, there's not that many people there) Congresspeople-look how long it takes for them to get stuff done.
It's better to have a framework (like our Constitution) which one can use to base future decisions off of. Having a small group of people represent a larger group (like our Congress) enables things to get done a lot faster. No, I'm not saying that the US has the best form of government.* It's still better than an absolute democracy, imo.
*The best form of government would be with me as Supreme Overlord of the Universe. I have experience. I've been playing the Civilization series since it came out
Marc