I think ranking is important in many respects, and totally agree with Greg Jennings assessment of a situation that does happen often enough. Actually, any comparision in my mind, is to the individual themself. How far have they come in their practice? What do they contribute on the mat and off? No two people are alike, and no two people will be in exactly the same place for their ranks. There is always someone who has outstanding ukemi, and always someone who suffers from perputally being a bit behind the 8-ball on their ukemi. Also, there will always be someone who is incredibly flashy on the mat (with no real substance), who everyone says "wow, look and JoeBob, isn't his aikido terrific" when at the same time you want to vomit. There will always be the very understated aikidoka whose technique is right on, but have no flash, and no one notices. Who is to say what is what, other than the teachers who are grading these folks.
As to teaching certificates and titles; it can the same effect as the rank issue.