Re: "super" high ranking yudansha?
My reason is personal. I'm curious. I hope that doesn't kill me.
Another thing is to have an almost authoritative reference. If there are about 600 6th Dans, it would make it easy for aikido newbies to verify that their senseis are "legit".
Now I understand that many many many good instructors are no longer affiliated with Aikikai (and I only mention this because that is the mainline.) And there are several "weird" groups that claim 8th Dan especially where I live (I'm in the Philippines.)
They're not really weird, but you know, it's been mentioned in other threads about these 8th Dans, and it kinda irks me or annoys me. I would not dare confront them about it either as I know they have been practicing aikido before I was born, but to me, that's just not right.
I mean, maybe this kind of thing can be brought up to whoever is an authority in the IAF. USAF lists all their promotions, so it's relatively easy to determine if your Sensei is on that list. (There are no USAF in the Philippines as far as I know; in fact, non outside the United States.)
For example, from what I gather, there is a list of famous aikidoka (in wikipedia), and in the Philippines I've met two of them personally. One is 2nd Dan and never been promoted and broke away from Aikikai, then broke away from that organization (because of politics he said) to start his own association. The other one is at least 4th, maybe 5th, but I can't verify and I don't want to ask him myself, although I know he is currently with Aikikai because he just sponsored a Japanese Shihan to conduct a 2 day seminar.
In this particular case, rank doesn't matter because their history is pretty well known and if you get the chance to train with them they will really help and improve your technique, and all the other 3rd Dans and 4th Dans in the country greatly respect these two.
Now, if it's a privacy issue, well - maybe have a voluntary wiki or database where everyone who has a Dan ranking input their info, willingly.