I agree with much of what you said, except the last part. Third party candidates are important because the two established parties are the same! Both are committed to increasing spending, bot are committed to keeping benign drugs illegal, such as marijuana, both support the PATRIOT act,, both advocate submitting our sovreignty to supranational entities like the WTO and United Nations. Both favor regulation of industry, much of which is unnecessary. Both oppose the First Amendment in the form of the McCain Feingold Act. Both supported the No Child Left Behind Act, which lets the fedeeral government dictate how your children are to be educated (and poorly at that!). Both prey on the fears of the electorate to enrich themselves at the expense of others. Both participate in pork barrel programs for their constituency. Both parties think the solution is more government, when it hasn't done anything right at all! (consider the crash of 1938, the war in Iraq, the Great Society, the current recession that was triggered by Greenspan monkeying with interest rates in 1999 and environmental quality) And finally, both are against the injection of third parties into politics! I have seen the last firsthand where the Ohio Secretary of State, Ken Blackwell, arbitrarily denied the Libertarian Party ballot access, since he knew our presence would siphon votes from the Republicans.
Neither party gives a damn about the Constitution or the Bill of Rights. They just twist them for their own purposes.
That's why it is important to have third parties: to kick the crooks and despots out of Washington.