From a Kerry canvaser who knocked on a thousand doors....
Undecided voters aren't as rational as you think. Members of the political class may disparage undecided voters, but we at least tend to impute to them a basic rationality. We're giving them too much credit. I met voters who told me they were voting for Bush, but who named their most important issue as the environment. One man told me he voted for Bush in 2000 because he thought that with Cheney, an oilman, on the ticket, the administration would finally be able to make us independent from foreign oil. A colleague spoke to a voter who had been a big Howard Dean fan, but had switched to supporting Bush after Dean lost the nomination. After half an hour in the man's house, she still couldn't make sense of his decision. Then there was the woman who called our office a few weeks before the election to tell us that though she had signed up to volunteer for Kerry she had now decided to back Bush. Why? Because the president supported stem cell research. The office became quiet as we all stopped what we were doing to listen to one of our fellow organizers try, nobly, to disabuse her of this notion. Despite having the facts on her side, the organizer didn't have much luck.
He doesn't address the question of whether decided voters tend to be any more rational, but I suspect not much, if at all.
This makes it even more amazing that countries with elected governments tend to be better places to live than countries with non-elected governments. The voters supply a tiny number of bits considering to have such a large effect they have, but I didn't realize how much low quality thinking goes into those bits.
On the other hand, I've believed for a long time that the main virtue of voting isn't that it gets the best leaders. Voting gets rid of the worst leaders, and a really bad leader (especially if their term in office isn't limited) can do much more damage than a good leader can do to make improvement.
Link thanks to Michael Vassar.