Robert Cheshire wrote:
I The only problem that exist is that you have to be very observant of the abilities of the person you are throwing and not throw with an advanced throw they don't have the ukemi skills for.
Certianly a "rank" system, regardless of how sophisticated it is (i.e. white, brown, black or 12 different colors) helps you to gauage the experience of uke and govern yourself accordingly.
In addition, it allows the instructor, especially in a large dojo, to gauge the experinece of the class they are teaching and craft the lesson plan accordingly.
Rank also helps to create an attmosphere of discipline, which is a major reason many people seek out martial arts training. While rank structure in many dojos isn't as rigid as it was originally meant to be when martial arts were part of the military, it still creates a structured environment, which sets a tone for training.
While O'Sensei always encouraged a "joyful" atmosphere in the dojo, martial arts techinques need to be practiced in a safe environment as well. Having someone "in charge" helps avoid the type of horseplay among junior students that can lead to injury.
Finally, having a rank system, even a simple one, that makes it easy to identify an instructor or senior student makes it easier for newer students to know who to go to if they have a question or problem.
These are just a few of my thoughts on rank.