The first major long-run impact is to push voter turnout down. But the dynamics of the ballot box are very different than the dynamics in city council or the state legislature. Telling people that they need to vote for someone who they don't really like or trust, or whose positions on the issues they have many disagreements with, feeds the cynicism in our democracy. That the election of Jesse Ventura had a much higher turnout than normal is proof that third party candidates can push turnout up. Further, it undermines the democratic process.
Voting is the one time most citizens take to express their opinion.
While I wish they would express it much more often, many don't. If they don't vote for the candidate whose positions they most closely agree with, we never learn what voters really think. Strategic voting to avoiding spoiling attempts to convince voters to vote based on who they want to lose, not who they want to win, or what policies they think are best. This is not democracy in action. Third parties have along history of being the engine of ideas in the United States.