A two-party system is necessary to pass productive legislation because it provides a form of checks-and-balances within the system. The potential for abuse when only one party is making all the decisions, is enormous. Within a two party system, there are big differences in views on what is best for the country. Input from both sides means that both sides get some of what they want and the public is better served.
There is a political gridlock in Washington because of partisan politics. The government shutdown is a clear sign that the two parties in Congress can't agree, even when it is of vital importance to the operation of the nation. Countries with a single part have less freedom, but their governments are run more efficiently.