Well my original reason was that I mark using the (my) AJAX marking block, which will soon find its way to the modules and plugins database. I click on the students name from the front page and get the single-question grading pop-up, but if they have written nothing, or gibberish (not that uncommon with some of them), to give them 0, I can't just press save, as there will be no change to the database and it will still appear as unmarked. My thinking was that a dropdown which started with 'ungraded' would be a better option as then it would differentiate between 'ungraded' and 'deliberate zero' and necessitate a db write.
On the other hand, I may have just been inelegant with my admittedly amateur SQL. Is there any db change that I have missed if I submit the question without altering the grade or comments?