How Important Is The Third-Party Vote?

Joe Trippi claims that Gary Johnson could "derail either one of the two major party candidates' shot at the White House." Dylan Matthews is skeptical:

[W]e can’t rely on polling, especially at this stage, but even later on, to predict how well a third-party candidate will perform. To see why, look at the Zogby tracking poll’s estimates of Ralph Nader and Libertarian Bob Barr’s vote shares in 2008. Nader averaged 1.75 percentage points in August; he ended up with 0.56, less than a third of that. Barr averaged 3.75 percent in August, with one poll giving him 6 percent, but he ended up with 0.4, even less than Nader.