there's no such thing as a wasted vote.
The election process is about more than just who wins. Sure, the winner is important, but there are other factors that have an impact on the behavior of government. For the sake of discussion, let's assume that one of the two major parties will win in November. Why vote for someone else?
A vote is a statement of your general favor for a given candidate. It's a winner-take-all proposition; you don't get to divide it among three candidates you like. It's assumed that you don't believe the candidate is perfect for you; he was just good enough to get your vote.
Voting for a third party or write-in candidate sends the signal that A) you care enough to vote and B) neither of the two major party clowns was good enough for you. To the extent that your vote matters at all, you have used it to tell the major parties that if their policies were more like the one for whom you voted, they might get your vote.
A vote for a third party encourages that party, and also the other minor parties. They see the number of people who voted for them, and know where their support is.
A vote for a third party lends them authority when they speak out. A press release from a party that got
But, it might be argued, doesn't that split the support for one of the major parties, causing the Most Evil Party to win instead of the Not Quite So Evil Party? Possibly, and that is part of the choice. Unless your tiny party is at one extreme of the spectrum occupied by the two majors, support for it will come proportionately from both of them.
Most people want to vote for a winner. To vote for a third party you have to get past that sense of wanting to be on the winning side and remember to vote your own mind. If you only vote for the candidate you think is going to win, you have effectively allowed someone else to vote for you.
Finally, voting for a third party encourages those who don't want to "waste" their vote that it's not such a waste. Voting is a herd phenomenon. When others see your party's vote total rising from past elections, they'll be more likely to vote that way themselves.