I personally don't see the benifit on time requirements on gradings. Each person is different and pick up aikido at different rates, I dont see the reason why someone should be held back if they are good enough to grade but haven't done as many classes as anyone else.
The way it is done with us, is if you think you are ready to grade you go along to a specialist course (a course aimed at your current grade), at the end of the course their will be an opportunity to grade if you choose. if you choose to grade and you meet the criteria for the next grade then you pass. if you are not up to scratch then you fail and get an evaluation on what you should improve.
as you go up in rank there are fewer opportunities to grade.
right up until dan grade, at which point you cannot request a grading, but must instead be invited to grade when you are seen to be ready.
I would personally prefer for people to be allowed to choose to grade to black belt and have the standard set high so if they are not up to scratch then they fail. As it stands that is not the case.