As a disclaimer, I am a registered Democrat, so what will follow is defense from a person whose ox will surely be gored. However, if small "d" democracy is about anything, surely fairness is high on the list.
John Deeth, who I consider to be a friend and ally, wrote in his blog that "[Ralph]Nader and [Cynthia] McKinney are expected to draw votes away from Barack Obama, while [Bob]Barr is expected to take votes from John McCain."
I don't disagree with John's assessment, but I do disagree with the presumption that Democrats and Republicans should be automatically given the benefit of being the one's that votes are being "taken" from. As it stands, the major parties already have a distinct advantage over any other candidate (tell me where on the Iowa tax form anyone can check off a box to make a donation to a third party?), plus election rules favor the two parties.
It seems to me that if anyone's votes are being taken, it would be these lesser known candidates. If the $$$ playing field were level, does anyone honestly believe that the plurality of the vote would go to any one party? It is interesting to me that anyone who is not brand D or brand R is often labeled as obstructionist (not by John, of course) to one or the other major party's chances for election. The fact is that the two major parties do not cover the gamut of ideas that people would like to see enacted and these third party and independent candidates offer those ideas.
Having said this, the result of third party candidates and independents on elections, as the game is currently played, is they can sometimes have an effect on the outcome. It seems to me that if the Big Two want to put the threat these candidates pose to rest, their candidates should do what they do to members of their own parties while campaigning for their party nomination; usurp the good ideas. It seems like a "win-win" proposition.