Let's face it, today most American voters will vote for the party they have always followed, there are some voters who vote for a democrat or a republican every at every election simply because it's a "family tradition" to vote for a certain party, putting more 3rd party politicians in congress would force the American voters to look at a politicians goals and history instead of simpily voting them in political offices by their party.
Typically, Republicans and Democrats are more at odds with each other than third party candidates are. So, third party candidates can potentially act as middlemen between the two parties. Third party candidates can also bring ideas to the table that Republicans and Democrats can't, and morph ideas given by opposing parties so that they are more palatable to those that oppose. Third party candidates help to balance out the bipartisan war zone, and so should be more represented in the Congress and the Senate.
Though some people do support there being more third-party candidates in office, it actually can cause problems in elections. One example is when Ross Perot ran for President. It is debated that candidate Ross Perot possibly took votes from then-President George Bush, making it easier for Clinton to gain the Presidency. Our country also operates on a two-party system, such as in Congress, so adding additional parties may complicate matters. Third party votes, additionally, are often throw-away votes, since these candidates rarely win.