One of the biggest reasons why we have just the "two" major parties is because they are terrific at playing off of everyone's fears. 'Vote for us, because if you don't the guys from the other party will ruin everything.' Never mind the fact that both parties will essentially do the same thing regardless, but at least it is "my" party that's doing it.
If people would vote their conscience, instead of the party line, this country would be much better off. Most people have become too indoctrinated into the "two" party mentality though.
For an education on multiparty political systems see entry at Italy. It's not pretty. The two party system is infinitely superior to one where the parties are so numerous that pluralities are never achieved. No thanks.