The thing is, if a third party gets enough of the vote, it gets the attention of the major parties. Remember Ross Perot's Reform party? They were the only one

The thing is, if a third party gets enough of the vote, it gets the attention of the major parties. Remember Ross Perot's Reform party? They were the only ones talking about the budget deficit at the time. Then, after eight years of Bill Clinton (of the party generally considered less fiscally responsible), there was somehow a budget surplus. An economic contraction and a tax cut then combined to eliminate the surplus, but it still shows that third-party votes can have power. Also, Federal elections aren't the only kind: third party candidates win elections with some frequency at lower levels, because at those levels the person can matter more than the party.