I was recently doing a Khan Academy course challenge when I came across a question I didn't know the answer to. Because there was no "I don't know" answer choice, I just had to guess from the answer choices that were there. And I guessed it right. But because of that, Khan Academy thought that I already understood the concept, but I didn't. As you can see, this is why adding a "I don't know yet" answer choice would be very helpful for me.
Other than that, I am enjoying Khan Academy so far