You're generalizing too much.
People vote for a plethora of reasons. Not everyone votes for the "best person for the job". I bet if you had access to the ballots, you could find a lot of straight D or straight R voters. I bet you could even find some that were straight independant, when the choice was available (damn the iconoclasts!). I seem to recall some news story not too long ago that mentioned a lady who voted based on whether she liked the way the name sounded!
Not everyone who votes takes it seriously. Just look for exit polls about whether people thought their vote mattered or was even going to get counted.
