Until a true third party has been built from the ground up -- until it can field a candidate who's earned national name recognition by rising through the ranks of that party -- a vote for a Third-Party candidate is a vote for the "mainstream" candidate you like the least.
If neither viable candidate really makes your nipples hard, surely one of them distresses, disgusts or just plain scares you more than the other. Your vote can either help stop him, or aid him. It does one or the other. Anything you do with your vote that doesn't bar him from office, works to _put_ him in office. And the only way your vote can help bar him from office is if it's cast for a candidate who can defeat him. Right now, there's only one of them.
Jill Stein, Ron Paul, Ralph Nader, however much you may agree with them, are not going to win the presidency. Obama or Romney is. Which of them would your third-party no-hoper most oppose? That's who you're supporting when you vote Stein or Paul or Zagnut.
Don't come whining to me about how it's not true, how your Stein vote supports Stein. That's just not true. Your Stein vote supports Romney -- or, if you hate him more, Obama -- by not helping to defeat him.
That's the hard fact.