It's a conceit - they call themselves "independent" when in reality they'll still reliably vote Republican or Democrat depending on where on the Liberal -> Conservative spectrum they sat their asses in or what party Mom and Dad or the boss belongs to.
Some do it because they don't want to admit to their friends and family who they really voted for. But they're still pretty much either a Republican or a Democrat already.
Hell, at one time I registered myself as "independent" just to keep the political parties from constantly calling for donations, surveys, and support during election season. I'd reliably vote Democratic; but I needed my home answering machine for work...
That was until the state went to a Dedicated Party Primary system. Then I had to register Democratic.
On edit - the most important thing to do is get those independents out to Vote - no matter what their politics. "Independents" are actually less likely to vote when it comes down to crunch time, because a good number of them are actually afraid of other people finding out who they voted for. Never underestimate the power of intimidation...
And more than anything else, do whatever you can to overcome voter suppression. That's how we get more Democratic voters.
Haele