I think that the ranking system fulfills a need in newer students to be able to mark their progress. While one could argue that an aikidoka should be above this, it does seem to be a fairly universal need. Once somebody has reached dan ranking, and proven that they are a serious student, I would think many would have outgrown this, or at least be attempting to outgrow this (as I am).
One could also say that dan rankings give a rough indication of somebody's qualifications to teach or run a dojo. If one just had dan rank and below dan rank this would be considerably more difficult, as dan ranks run from shodan, which to me indicates just a serious student who has some knowledge of the basics, on up.
Anyhow, that's my two bits.