Rather than rehash any previously made points, I will simply add this to the mix (many of my beliefs echo statements already made).
Rank only has the significance that is assigned to it. It is assigned by the student, the instructor, the organization, etc. While there are some perceived notions about the "black belt" from the outside world, I have been part of dojos and organizations that view the rank anywhere on a large spectrum, with the extremes being:
1) Someone who has spent a year or two learning the basics and is ready to learn the REAL system and art
2) Someone who has mastered the art and can easily and effectively teach it and share (10-15+ years!)
One can debate the value of either, or the views in between (of which most schools would fall into and there is obviously a wide range). However, it is important that the integrity of the rank be maintained within that dojo or group. If I say Black Belt equates to general mastery of the art, a student should understandably be upset when someone else is promoted to that rank who obviously does not possess the skill that he has been working toward. If i define it purely based on time in rank and/or mat time, then I have set the expectation that X-number of hours = X-rank, and when you reach that milestone, you'll want the belt, and, you'll be upset if you don't get it when you reach those hours OR if someone else gets the rank BEFORE they meet those requirements.
Whatever the requirements are, they are right for that group, but must be held to. Rank is a human construct and completely arbitrary - the individuals provide the meaning to the rank, but once you violate that meaning as an instructor or organization, you should expect dissent and complaint.
I have left schools or organizations over rank, even though rank itself was not the reason I trained. It was a catalyst and a symptom of other problems with the organization, and it was definitely a contributing "final straw." The main reason? Rank had been established and requirements expressed, but those were changed for commercial reasons and became inconsistent. Again, it was the inconsistency that prompted the dissent. If you change your policies, that's fine, but we had all better change together.