Sarah Palin =/= President.

An Independent~ Third party there just to steal votes
An independent~ Affiliated with no party, first, second, or w/e

On a presidential ticket, you're right, independents don't do well. However, if you look at someone like Joe Lieberman, who got tossed out of the Democratic party and then won his senatorial position as an indepedent, that's not always the case. In local elections, independents get elected all the time.

Also keep in mind the case of Ross Perot, who dropped out of the race a few times, and got 22% of the national vote anyway. In the end, it's really not whether you're Democrat or Republican, it's how much money you have. The two parties have the money, that's why they do well. If some 3rd party were to be able to throw 10 billion dollars down for a presidential race, there's a good chance they'd get elected, provided they were somewhat mainstream.