Nader's 97,000 votes in Florida represented the difference between winning and losing Florida for Al Gore, and Gary Johnson could tip Arizona/Colorado/New Hampshire/New Mexico, where he's polling 9%/7%/7%/13% (if the pollsters are including him).
Unlike Nader, Johnson could hurt either of the 2 major-party candidates. It's more likely that he'll hurt Romney, but possibly not in Colorado and New Mexico. He could draw off erstwhile Obama supporters. If you're in a swing state, you have to want to make the point of giving no help to either of the 2 individuals who have an actual shot at winning. It's not enough to simply prefer Gary Johnson. (Take the I Side With test and see if you do align with him.)
Who are these people who not only want to say I prefer Johnson but also I decline to have any effect on the outcome? In 2000, the people who voted for Nader — did you know Meade was one? — perceived no significant difference between Al Gore and George Bush. Many of them went on to view that perception as quite inaccurate.
It's hard to believe the potential Johnson voters will think like that about Obama and Romney, which is why I think swing staters won't go third party this year. But I think the presence of Johnson in the race might affect how Obama and Romney fight in the swing states. Johnson is there as a refuge if the candidate we feel we're supposed to vote for offends us.