I understand that not having an "I don't know" option could potentially inspire the examinee to give a question an amount of effort they otherwise wouldn't. However, my desire for this option is due to the fact that I take course challenges and unit tests frequently just to evaluate what I do and do not know on the topic despite not having touched the topic in many years (i.e. never having done any exercises in the course/unit). See, I'm 24 and am finally severely disappointed in how public education failed me enough to now spend a lot of time going back and learning for real all the things my high school diploma says that I learned and in fact did not.
So, there are many questions that I'm given which I do not want to have a chance to accidentally get correct, because I know I do not have any idea what it's talking about, so I need the examination to accurately show me what I need to work on. When I get something right due to pure chance, I may forget which topic that was by the time I finish the examination, and then it just says that I'm Familiar, when it should ideally say Attempted instead. I could click the Review related button and bookmark the lesson, but it would be way better if my circumstances (likely shared by many others who love this site for this purpose of re-education) were accommodated by the feature set itself.