Any rank belt that is earned represents a level of knowledge - no matter what the rank is, it says that someone, somewhere, is certifying that the wearer has a certain level of information, and the ability to demonstrate that information, in a particular style.
Black belt, to me, is a beginning - a sign that the student has learned to control his/her actions and reactions, physically and mentally, enough to perform at a certain standard, and is ready to begin the next level - training as a black belt, instead of training to be a black belt.
For many people outside MA, black belt is the ultimate rank - people with little or no knowledge of MA often don't have the information necessary to understand the differences between levels of black belt. Many people start an MA with the goal of black belt, not really understanding what that goal means - and too many reach their goal and are unable, or unwilling, to set a new goal, of continuing to learn and improve for the sake of learning and improving, which is, I think, why so many people drop out after black belt. Rank will come, in time, if one continues to learn and improve - but as far as I'm concerned, if your only goal is rank, you are missing a major piece of the journey.