I think that you should vote for Barack Obama if you think he’d be a better President than Mitt Romney, but that you should instead vote for Romney if you think he’d be better than Obama. If you think that there’d be literally no difference between the two of them then I think “tie goes to the challenger” is a reasonable principle. But under no circumstances does it make sense to vote for Jill Stein or Gary Johnson.
That said, forecasting the long-run impact of particular elections is very challenging (one can certainly tell a story in which left-wing defections throwing the 1976 election to Gerald Ford would have led to a more progressive long-term outcome) so people are going to disagree.
But I’ve noticed that various anti-Obama pro-third-party arguments on the Internet proceed with an annoying two step. Usually the headline and the lede of the piece will be very focused on Obama, the evils of Obama, and the braindeadness of the Obamabots but then the argument will employ as a lemma something like it doesn’t matter who you vote for because your vote won’t make a difference anyway. I think that math is more contestable than people often realize but whatever you make of it, if your argument is that it doesn’t matter who you vote for then that’s an argument about voting not an argument about Obama. If it’s true that you shouldn’t feel constrained to choose a major party candidate on the grounds that your vote won’t swing the outcome anyway, then the exact same conclusion would hold even if Obama had cracked down on banks much harder or never bombed a soul or delivered single payer health care or whatever you like. The argument may be correct, but it’s an argument about an entirely different subject.
I also think that as an argument written for public consumption on a well-traffic blog or website (as opposed to simply offered over drinks at the bar) it’s an illegitimate form of argument.
“Why I’m Voting For Jill Stein” or “Why I’m Voting For Gary Johnson” is, qua article, an effort to persuade other people to do the same thing. A persuasive argument that takes as one of its premises its own failure to persuade is inherently problematic.
Articles that posit some kind of difference in behavior between residents of swing and non-swing states are even worse in this regard. In 2004, John Kerry came within about 60,000 votes of carrying Ohio and the election. But he also got less than 55% of the vote in California, Oregon, Washington, Minnesota, Wisconsin, Illinois, Michigan, Maine, New Hampshire, Connecticut, New Jersey, Delaware, and Pennsylvania. Which is to say that an even modestly successful effort to organize third party defections in non-swing states would rapidly turn the majority of those states into swing constituencies.