If you consider the Tea Party movement a minor party, I would say the answer is yes. This is a small, fringe movement that managed to shape the national discussion in the last election. They also got some of their members elected. If you look at them as more than an offshoot of the Republican party and a party themselves, this is quite a feat.
I agree that minor parties help keep either of the two major parties too powerful. They do bring out some of the issues that the other two parties would prefer not to address. In addition, minor parties sometimes split the voters and take away some of the power of the majors.
I think that the minor parties serve as a check and balance for the two major parties. They bring to light issues that the other two parties may not be willing to give any attention to. As someone else has already mentioned Ross Perot put the deficit in the public eye and forced the two parties to address the issue.
Minor parties do contribute to the process in important ways. Sometimes these small parties, with only a few percentage points of the vote, can become "Kingmakers". That is, the election between the two major parties is so close that one of them is tempted or forced to adopt some of the minor party's issue positions, or to include some of their candidates in government in return for their general election support. This is more common in parliamentary systems in Europe than it is in the US, since a third party rarely starts here unless there is real dissatisfaction with both major parties. The Tea Party, for example, in this last election, was mostly unwilling to compromise even with Republicans.
I would argue that minor parties can contribute by bringing issues in to the public eye that would not previously have had much salience to the general public.
For example, the candidacy of Ross Perot in 1988, especially, brought the issue of the government deficit into the spotlight much more than it had previously been. You might also argue that the Tea Party has done the same for government spending recently.