I think color-belts also provide a very important role. When a senior instructor goes into an area he has never been, or with people he has not met before to give a seminar, this provides him / her an idea of what the level of ukemi is of those folks. So when he / she works with that person, they won't assume then to be a certain level where they are not.
I would agree if there were a consistant, universal standard for rank
. I would submit there is not among different organizations and would further submit there is not even within the same organization.