When the United States was first getting started and all the laws were first being made the citizens were afraid that the government was going to become too powerful. Therefore they wanted many different people to be running the government instead of one person or one small group of people. This added to the fear of having political parties. If one political party had control of the government then it would become an oligarchy in a way. A central authority was something to be distrusted, watched, and curbed.

It wasn't necessary to be a part of a political party back when our forefathers were writing up the Constitution and forming our nation. George Washington didn't label himself as a Federalist or a Republican and he was the only president unanimously voted into office by the Electoral College. Today this is not possible. The Republican and Democratic parties have become the most influential today and have money to help them on their campaigns. If you wish to run for office today you are expected to be from one of those two parties, or, unless you are like Ross Perot, pay the money for your own campaign and not have as good of a chance at winning an election or stating your disapproval.

The question on whether or not they should be accepted as a legitimate means to express political disagreement is really up to the citizens. As long as the parties don't become too powerful then there shouldn't be a problem at all. If somebody wishes to express a disagreement and they don't agree with either of the current two parties then by all means let them start their own party. As long as the citizens agree with what you are saying it shouldn't really matter what political party you belong to or what political office.