The article linked below argues that, despite media coverage touting the importance of independent voters, (a) self-identified independent voters are not really independent, and (b) they do not have a significant effect on close elections.

<< It's true that independents are a diverse group. But that's mostly because the large majority of independents are independents in name only. Research by political scientists on the American electorate has consistently found that the large majority of self-identified independents are "closet partisans" who think and vote much like other partisans. Independent Democrats and independent Republicans have little in common. Moreover, independents with no party preference have a lower rate of turnout than those who lean toward a party and typically make up less than 10% of the electorate. Finally, independents don't necessarily determine the outcomes of presidential elections; in fact, in all three closely contested presidential elections since 1972, the candidate backed by most independent voters lost.

[snip]

<< In 1976, most independents voted for Gerald Ford but Jimmy Carter won the overall popular vote. In 2000, most independents voted for George W. Bush but Al Gore won the overall popular vote (despite losing the Electoral College). And in 2004 most independents voted for John Kerry but George W. Bush won the overall popular vote.

<< In a close election, a candidate with an energized and unified party base can sometimes overcome a deficit among independent voters. That doesn't mean the candidates should ignore independents, but it does mean that unifying and energizing their own party's base is just as important as appealing to the independents. >>

Source:
http://www.centerforpolitics.org/crystalball/articles/aia2011070702/