Rank is just what it is - nothing more, nothing less.
Ahh, but what is it? That is the real question.
Some seem to look down on the concept of ranking.
I don't have any problems with the concept
of ranking. We rank things all of the time. Any time you make any kind of decision you have to first rank the possible choices according to some criteria.
The problem lies in how that concept is applied
within aikido organizations. What are the criteria for ranking people in a non-competitive art like aikido? Every group has some sort of list of techniques that must be performed, but beyond that there is no specification of how they must be performed. Typically, they are done on a fully compliant, non-resisting partner of sufficient experience to know the ukemi well enough to make the nage look good. Rather than requiring some objective level of skill, aikido tests are usually look more for how well someone conforms to the particular style of their organization.
The other formal criterion is practice days, which usually have to be within a dojo in the organization you are testing in. This is more a measure of loyalty than anything else. And of course, given the subjective nature of performing techniques with a fully-compliant partner, often for one judge who will ultimately make an entirely subjective decision on his own, politics factor heavily in all aikido promotions. In fact, above 3- or 4-dan even the pretense of testing is done away with in most aikido groups and all aikido promotions become political.
So rank "is what it is," but what it is in aikido is not what people think it is. It is by no means an objective measure or guarantee of skill, even within one group, and certainly not within the general martial arts community. Nor is it a necessary or even sufficient requirement to teach anyone anything. It is nothing more than a measure of one's political position within one particular organization.
If people understood that and did not try to make rank into something it is not then I suspect there would not be so many problems. However, I think the organizations actively encourage a misunderstanding of what rank is because it increases their power over their members when people consider political status (ie, rank) within an organization to be more important than actual skill, as many, if not most, people in aikido do.