I don't buy that logic at all. Your vote should be an expression of where you stand on the issues that you care most deeply about. That's what democracy is supposed to be about, not triangulating your positions in accordance with the mathematics of a corrupt system. A vote for third party is not a vote for Romney. It's a vote against this corrupt and stinking status quo, of which Obama is a part. Until we get the special influences out that corrupt our government, it's only going to get worse. As long as people continue to play the two party rigged game, it will continue.
Since the 80s I've voted for a lot of Dems. What have they done over that timespan? Gone further and further Right. Gotten further corrupt. Hell, what does the Democratic Party even stand for anymore? They've triangulated so far that their identity has dissolved.