A late comment from me (I blame the holidays).
The only real correct answer here, as is obvious from the comments already posted, would have been "it depends".
In an ideal world, I would agree with answer 5; in the real world, I think this is not always correct. answers 1 and 4 come close, but not really correct.
My personal answer is a combination. In the role of answering questions, the value of the QotD to me is a combination of judging my knowledge and learning. That's why I always read the discussion after answering, whether I am right or wrong. And the QotD also very often makes me look up stuff I didn't know yet - both before and after answering.
In my other role (as a contributor), the QotD to me is about sharing my knowledge. There are more ways to do that - I blog, I speak on conferences, I (sometimes) reply to questions on forums, and I sometimes write a QotD - when there is something I want to share that lends itself to this format.
And of course, submitting a QotD and then reading the comments is also a great way to build a very thick skin.