The Two Party System

Since we have entered election season, I was reflecting on my vote in 2004. For full disclosure purposes, I voted for Nader. I was thinking about the multiple complaints I received when telling people about my vote. The consensus was that voting for a 3rd party candidate was "wasting" a vote. The more I reflected recently, the more I wondered if they considered an abstention a wasted vote. Does the fact that Americans live in a two-party political system make them responsible to vote for the lesser of only two evils?