From a ranking point of view, organisations can be quite important. Imagine qualifications in other skills, such as medicine. Although I°«m sure there are some excellent °»bush doctors°… out there, saving lives in volunteer clinics every day using just a coat-hanger and gaffer tape, given the choice, I would want the surgeon operating on me to be certified by a recognised authority. It°«s still no absolute guarantee of competence, but there°«s a good chance the guy won°«t be a quack.
I'm not sure why people keep coming back to the idea of martial arts ranking being in any way like professional qualifications. They are different in very important ways.
The AMA has a monopoly on the qualification process for doctors of allopathic medicine in the US. This process is open, rigorous, and competitive. You take a competitively graded exam to get into an accredited medical school, pass a competitive admissions process, then take and must pass many courses taught separately by independent instructors and also graded with competitive exams before you receive a MD degree.
Martial arts organizations have no central qualification process but each establish their own guidelines. Imagine if there were a dozen different medical associations in the US each claiming that they had the authority, through lineage, skill, or any other claim, to authorize doctors to practice their "style" of medicine. And then imagine that these organizations do not competitively test their entrants but simply allow the tester discretion to individually pass or fail each applicant on any grounds, with no explanation. Now imagine that they do not even have any open guidelines for their MD degree but just choose who they will award them to by recommendation. Would you want to go to a doctor that had a diploma from that kind of school or one who graduated from an AMA-accredited school? It's not a tough choice, is it?
Now if I want to study an art like kendo then sure, ranking is something I will look for. In kendo, every rank is tested for based on centralized, competitive standards and the goal of organizations like the IKF, I suspect, is to establish a standardized kendo that is taught more or less the same everywhere in the same way that McDonalds tries to have food that tastes more or less the same no matter where in the world you buy it. It's a very modern approach to martial arts, and if you want the "taste" of IKF kendo, then look for rank from IKF-affiliated groups. Simple.
The contrast to this model is the koryu-ha, which are organizations designed to limit and restrict knowledge and thus to maintain an unique, local "flavor". I think aikido is fascinating to look at and also quite problematic because it tries to combine these two very different models. This is particularly true in the aikikai, which allows a great deal of freedom for its affiliates to follow their own style under some general guidelines. When you look at people like, for example, Kato and Chiba, both direct students of the same man, their styles are so different that it's almost as if they are doing different martial arts.
Perhaps groups like the Yoshikan and Shodokan try to establish a more standardized practice throughout their organizations, but even here these are quite different attempts by direct students of the same man as the other two trying to take their own interpretation of what they learned from him and standardize it. The interesting thing about the aikido Moriehei Ueshiba taught is that it seems as if it was never meant to be standardized, which I something I personally like about it. The problem is that this makes it a very bad martial art for building an organization.
The aikikai has tried to walk a fine line between maintaining the spirit of allowing individual "flavors" of the art while still trying to establish enough general standards (or perhaps just the illusion of them) to build a worldwide organization. One problem is that these standards are in no way clear or rigorous enough to support a ranking system that means anything, so you have an aikikai shodan, for example, that is completely different in pretty much every way you want to look at it depending on where it comes from. The idea that belonging to such an organization or having qualifications from them is a very good measure of credibility is very much questionable.
Again, I'd be more inclined to look at rank as an important qualification in other organizations that try to create a more standardized curriculum, but that assumes that I'm only looking for that particular flavor. And even those standards are no where near as rigorous as those of an organization like AMA, so there's still no meaningful comparison there, nor does there need to be. Martial arts are just a hobby that people do for fun. Nobody's life is at stake, and if it is, they shouldn't be getting their primary training studying a traditional martial art.
Organizations, including the aikikai, have been very successful at making this art available to a lot of people who might not otherwise have any exposure to it. I think it's important to respect that. However, this does not mean that these organizations should be accorded the kind of blind loyalty that many who belong to them often offer, generally at the encouragement of those very same organizations who are competing with other organizations and independent teachers for money and student loyalty. Organizations should serve people, not the other way around. Use them if they suit your purposes, discard them if they do not.