Not necessarily. When the two major parties cease to be a check and balance on each other with regard to this or that issue, the threat of a third party insurrection is sometimes the only means of chastising one's own party, and evoking in party leaders for certain constituencies.
It's not a card that should be played frivolously, or often, but it certainly has utility.
When leaders of either major party ignore the prospects of a third party insurrection, and end up losing elections, they have only the myopia of their own vision to blame.
On the other hand, I trust that you acknowledge that it's also a popular tactic to abuse the above said card, even going so far as funding a Green or Constitution or Libertarian Party in order to merely syphon votes away from your competitor.
Bravo.
Not only that, but politics is more than a "us VS them" sports analogy.
People go to ball games and such to cheer their favorite team, and pray the opposition loses. People become interested in politics however, because of issues.
When both major parties refuse to take up a particular issue, what are the supporters of said issue to do?
It's like someone who wants mexican walking away from Burger King & McDonalds, to the Taco Bell down the street.
Logically, if the two former businesses didn't want to lose their customer, they might try modifying the menu options to accomidate him, and earn his sales dollar fair and square.
Instead, what we are seeing now is the management of the two burger franchises taunting the man, blaming him for his taste in food and calling him a bad customer.
In any situation than our two party monopoly, this strategy woudl fail miserably.