It's not really a myth.
Yeah, theoretically, if enough people voted for a 3rd party politician they would get elected.
Buuut.. let's be realistic. How likely is that to - ever - happen? Ron Paul didn't even come close when he was running as a libertarian; and when he switched to the republican party so he'd have a fighting chance (at least, I assume that's why he switched), he was crushed.
No other third party politicians even have the ability to make their name known. No money, no status, no foot in the political door.
And unfortunately, those are the things you need to be seen and heard on the political stage.
Third party politics are relatively pointless.
We technically have more than two choices, but in actuality, the race is between two parties.