I understand that it's optional. But if this type of thing becomes popular, programmers will feel pressure to show their employers an online track record. People will feel obligated to participate, even if they don't want to, and those that do participate will worry a lot more about how their answers will be interpreted by potential employers. The penalty for an incorrect answer becomes much higher than just a downvote.
Just my thoughts -- I'm speculating here, and there's still a long way to go.