Why have a ranking system in Aikido? Most other MA hold certain info. at certain ranks and you can't practice that material unless you achieve that rank. But in Aikido we all practice the same things, over and over, nothing is withheld (as far as I can tell) and you're free to ask any question at anytime and get material. Everyone just pracices from exactly where they are--which is all you can do anyway. So...what's the point?
Martial arts at a certain point in Japanese history were not for everyone: most didn't have the time, the money, or wanted the obligation that went with the practice. The ranking guidelines followed very closely to the artisan-apprentice model that had formed the cultural bedrock of "Old Edo" society. That is, the apprentice spent many years with the "craftsman" learning through a slow process of osmosis every little detail of the craft. It took a long time, but imbued the student with a very exquisite and competent ability that most people in Western society aren't aware of.
After the removal of the class system propagated by the Tokugawa shogunate, people had more options. Technology changed, assumed a more Western bent. Education did too. Jigoro Kano introduced the kyu-dan ranking system in his new form of jujutsu as a way for the "new society" to assimilate the budo using Western educational methods, having an objective syllabus and an honest criteria devoid of class interest. Other "new" martial arts (gendai budo) followed suit, including Aikido.
Aikido did not always use the kyu-dan system. Before WWII, most students received densho like other koryu; after the war, they received kyu-dan ranks. Aikido had a different training philosophy, relying less on objective criteria and a Western approach, and leaning to a more mystical and "Founder-driven" approach. This "Founder-centric" model persisted in spirit, if not in actual reality with Ueshiba's students who have now become teachers. The then-students-who-were-in-charge made up the criteria for the younger ranked members, making sure rank was being passed along as it should, but this criteria was never formally defined by Ueshiba. It all revolved around and depended on him, so that as his aikido changed, so did the methods his students used to teach it and the criteria they used to judge competency. As people like Gozo Shioda were approached by the Tokyo Metro Police for instruction, the method changed to fit the audience. The same applied to Tomiki at Waseda University, and so on.
As far as I know, the idea of showing/instructing all techniques regardless of ability or rank is unique to Aikido in the Budo world, and is a direct translation of the Founder's educational method. Whether or not it is effective is up to debate, and is a touchy subject to some. What's the point? Make up your own, I guess. It is kind of interesting though.
My own take on Ueshiba's motivation is not that he simply wanted to be the head of a cult of personality (though there is a lot of evidence to the contrary that supports his egocentric nature), but that he was trying to change Japanese society and make it more "free" much the same way Jigoro Kano was, but utilizing the religious and martial insights he had gained from his teachers. Perhaps he was looking for a revolution from within the culture rather than an assimilation from the outside world.
I know I probably didn't answer your question, but then again, who really could? We all take what we want from this practice based on our own motivations.