Sorry, I don't intend on hounding you more or unnecessarily prolonging the silly argument just because I ever so slightly disagree with this, but I feel like there is one thing that pertains to this and holds more importance than some people might think it does.
It is the fact that even though strictly voting against someone solely because they have already won the tournament is kind of silly, for the people to whom that is a valid argument it also is a distinctive feature of a character that they take into account when weighing the two voting possibilities in their heads. This may or may not, depending completely upon the factor of individual taste when it comes to deciding which of the characters to pick, have bearing on the decision. If you feel, like I have felt a couple of times this year, that you'd rather vote for someone else because his or her opponent is a previous winner, then you do. And isn't it the feeling that counts? Isn't it, on grassroots level, what makes us feel good about voting and participating in the first place? To follow your own feelings and stand up for them in front of others.
I daresay that in general it's not so much about silly dogmatism than it is about making the very same "I shall have fun this way!" decision everyone else does. The most of us just can't put it in any other words that would probably better express what goes through our minds and emotions at the moment of pressing that vote button.
Ugh. Dryish, you are seeeeeriously overthinking it.
People can vote for whoever they like, for whatever reason they like.
And similarly, people can think voting for a character for certain reasons, is a crummy reason. Kinda like what Gekko said.
You can't make the argument that people are free to choose their reasons behind voting without acknowledging that people are also entitled to dislike said reason for voting, and choose to vote differently.
Neither viewpoint is wrong.
So vote for who you want to, dude. No one is stopping you. :/