A two party sytem is good because it provides options for the voters. In a one party system, the ruling party would have a monopoly, and it could basically do whatever it wants. A two party system would have more competition, so that both sides of an issue could be reprsented.
Having only two parties is ridiculous. You are forced to coincide with mainstream view you if you have nonmainstream ideas. Prevents change. Blocks new parties (sometimes quite literally. Laws for elections are dictated by the two parties in America, look it up ) prevents new laws being passed. Causes scapegoating, the "damn liberals " effect. Only in America
Because the two party system creates a continual disagreement between two sides, and most people fall somewhere in between anyway, the two party system neither accurately represents the American people nor is the most efficient way to conduct politics. True democracy, where everyone actually has a vote on every issue, would be a better way now that technology would support it.