Thomas Jefferson had this idea that the Constitution should be torn up once per generation (which, being Thomas Jefferson, he used actuarial tables to calculate at 19 years).
This idea terrifies me, by the way, because I wouldn't want people like Rick Santorum or Al Franken writing a new constitution, since their ideas of personal liberty are horribly, horribly at odds with my own.
Our Constitution was written in 1787, in a three-mile-per-hour world. It was pre-Freud, pre-Darwin, pre-Einstein, pre-germ theory, pre-atomic theory. It reflects the values and the times that produced it.
So, if you were writing the New Constitution in the year 2012, what would you add or take out? Or would you scrap the whole thing?
By the way, I'm going to preemptively address two reactions to this:
1. We wouldn't need to rewrite the constitution if we followed it as written.
Fine. That's my view too. But if you're going to say that, keep in mind that that's Ron Paul's position, and it's a degree of libertarianism you may not be comfortable with. It's not just ending the Drug War, it'd be eliminating things like the USDA, the FAA, the CIA, the FDA, Social Security and the FBI.
2. The Constitution was handed down by God/is divinely inspired, et cetera.
Believe that if you wish, but it doesn't further the discussion. The men who wrote it certainly didn't believe so. And not everyone believes in your god, so it may not be persuasive to them.