More often than not, third-party candidates have little effect on the ultimate result of an election. The eventual performance of most third-party candidates is lower than the margin of error of most polls (2%-4%), and so a poll that includes every single candidate running in the election (yes, even the Rent Is Too Damn High Party) provides no useful information about the likely results of the election. But when a third party candidate begins polling well (above the margin of error), pollsters would do well to include that candidate in their survey questions, so as to more closely reflect the range of choices voters have. A poll that gives a choice between Al Gore, George W. Bush, and Ralph Nader, for example, could give a different result from a poll that gives a choice between just Gore and Bush.
So in an election like the Senate race in North Carolina, where Libertarian Sean Haugh has been supported by about 10% of respondents on average in the polls that include his name, it'd probably be a good idea to continue putting his name in polls, since even considering sampling error Haugh's performance is probably fairly strong. Certainly it's strong enough to possibly tip the scales in favor of either candidate in a close race like North Carolina.
