Agreed. There is an accountability which naturally happens when two groups of different ideologies are watching each other and holding each other to their word and pointing out their misdeeds or wrong-headed actions. Now, that often goes way too far, of course, and can quickly turn into simple finger-pointing rather than moving to anything more constructive. The divide is deep right now, but there is still a sense of accountability and a balance of power when someone else is watching every move. Ideally, voters should be doing this and voting out those who are not representing them well; however, we've become awfully complacent and uninterested, it seems to me. So, it has fallen to the parties to do the calling out and oversight. The good news to me is that these things are still our right, and voters can take back that power, which was given by default, at any time.
As we have a two party system in the United States, we enjoy the advantage that there is always a group in power, and a group to criticize the party in power. Criticism, while not always constructive, gives the public alternatives to vote for, and gives the party in power an incentive to work for the popular will. It's a form of competition in a democracy, and therefore the people benefit.
There are drawbacks at having only two parties like we do in the US. Minority viewpoints and organizations have a hard time influencing political policy themselves, cannot get enough support to start a third party and are thus shut out of the process when many of them have something positive and constructive to contribute to our democracy.
The benefit of rival political parties is that a system of checks and balances are created. It forces compromise. It forces shared-decision making. It forces work to come to conclusions that will work for all people, not just the majority.
In our country these rival parties experience different moments of control over the country. Right now, the Democratic Party controls the House, the Senate, and the White House. Elections occur every two years and the people get to hire and fire as a group by their votes. If they approve of Washington's Dems, they will keep them at work. If they are mad as all get out, you will see a mass firing.
The reason for this is that you have to have various options for people to vote on in order to have a true democracy. Imagine if your options in the next presidential election were Pres. Obama and Pres. Obama. Or if you had a choice between Pres. Bush and Pres. Bush in 2004. In what way would that be a democracy?
When you have rival parties, the people get to choose between parties with different ideas and different policies. This is good for democracy because the people get to help rule by choosing between these options.
Rival political parties is an important and essential requirement for effective functioning of any democracy. This is for two reasons. First, it is essential to make the power of people to choose their representative a rel power. Unless the voters have a alternative candidates to choose from the freedom to choose or vote for a candidate of their choice has no meaning.In a single party system the party chooses the candidate. Thus the real power is in the hands of the people who control the party rather than in the hands of people.
Second, the opposition plays an important role in controlling the activities of the ruling party in power. Without any opposition to examine how the ruling party is managing the government, and to point out to their shortcomings, the ruling party in power can become careless and make mistakes. Also, in case the ruling party shows any signs of usurping the powers of people, the opposition parties provide a readily available defence mechanism to organizes people's opposition against the offending actions of the ruling government.