I feel rankings in the Mudansha are relative simply because of interpretation. Obviously as John Lennon has said "Life is what happens to you while you're making other plans." Some Aikidoka have great skills in some areas, lesser skills in other things. This is taken into account because they are Mudansha.
After 20 years of study, I would expect greater things of someone who had studied for 5 years, even though they are both Yudansha. Thus ranking serves more to the student in knowing who to watch for correction of one's own technique.
Even so, I have had Sensei ask me which Nikyo I liked better, hers or Sempai. DANGER! No right answer! Sensei is very polished, Sempai very strong - both work! It makes no difference on what rank slams - er eases me to the mat, I am still Uke or Nage and will still practice to make myself a better Aikidoist. That to me is more important than the rank I have earned or the recognition I am given.
Mike in Denver