This is the same reason I always vote against the board of directors for any corporation that I have a vote for. I know the vast majority of people simply vote yes because they want to believe that their faith in the nominating committee is well placed. If there happen to be legitimate concerns, the informed objectors have a huge chunk of ignorant yes votes to overcome. A few ignorant no votes helps to even the score in such cases, while not really impacting the outcome when an election is truly uncontested.
Changing the default for organ donation won't alter my answer (yes), because it's not quite the same scenario from a game theoretic perspective. The rightness or wrongness of an overall outcome isn't really at issue, but rather only the individual's preferences. What is really being impacted here is the average outcome from those people who are truly indifferent, lacking in self awareness (regarding their preferences), or simply lazy. Anyone who cares about the answer is not losing anything in this.