The current poll asks if we think belt testing is good for aikido. I voted no, but I'd rather have voted "maybe".
Belt tests have been an integral part of aikido all over the world since the days of O'Sensei and his uchi-deshi. The formal belt-testing ceremonies have been apparently effective for years. I dislike a hierarchial system in anything, but I think students need to know the difference between a sensei and a white-belt.
The problem of sempai/kohai occurs when a person might have missed a test or two, but has been doing aikido longer than one of a higher rank. I believe aikido has a certain horizontal aspect to it (at least within the kyu ranks.) I think formal belt testing for kyu ranks is ok, but I think there is a better way:
A sensei sees his students each class and gets a very good feel for what rank his/her students should be. I am a 5th kyu, and I believe that's where I should be, not because I missed tests but because I missed many classes and did not advance.
What if the sensei just said after class, "Congratulations, Person X, I feel you have progressed much in aikido and you are ready to be an Xth Kyu. The only downside I can see is that this might make other students jealous. Jealousy, however, is a worthless emotion and no true aikidoka should possess it (or be possessed by it.) Just some thoughts from a lowly Gokyu, take it or leave it.