The fallacy in the lesser-of-two-evils argument is the assumption that a vote has an impact only in the election in which it is cast, when in fact a vote has more of an impact AFTER the election. When politicians get elected, most of them don't behave so as to serve the people who just voted for them (that's over); instead they behave so as to attract votes in the NEXT election, using votes cast earlier as a guide to where the votes are. While your vote will rarely decide the outcome of an election, it will always have this later influence regardless of who wins. The only way to pull the candidates who do win leftward is to vote for people to their left.
Another post-election effect of voting for third parties is that it influences more people to decide that doing so is worthwhile, once they see that other voters are increasingly crossing over. As long as most everyone continues voting for the two major (corporate) party candidates, the potential winners in each race will continue to be two very similar candidates indefinitely (primarily serving corporations and their owners).
Besides, voting for somebody that you actually like is the way this thing is supposed to work, right?