"...Third-party candidates will only have limited appeal and can only serve as spoilers."
Very true, third parties are only viable in parliamentary systems. In our system, whenever a third party has arisen it either faded away (Bull Moose 1912) or replaced one of the 2 main parties, e.g Republicans and Whigs in 1860.
In 1912 TR challenged Taft for the in the general election after he felt the nomination had been stolen from him, probably true. The Bull Moose polled more votes than the Republicans, but only swung the election to Wilson. The GOP leadership blamed TR for the debacle, but in fact they were as much to blame for their dishonesty. In 1916 TR supported the Republican candidate, who almost won, and the crisis was over. In fact the guy went to bed as the winner, since he had the electoral lead but California tipped it to Wilson.
http://www.suite101.com/article.cfm/presidents_and_first_ladies/41238
In Canada about dozen years ago the Conservative Party went from controlling Parliament to holding 2 seats, in a single night. That is a parliamentary system, but the results should serve as a caveat to any party that takes its base for granted.
http://en.wikipedia.org/wiki/Canadian_federal_election,_1993