In our dojo, it's sort of a defacto standard that after your first test you should invest in a hakama. I don't know how this came about, I guess if you stuck with the training long enough to test, chances are you'll probably hang in there....
One thing to think about is that the hakama is actually a training tool as well. How many of you ate it big time the first time doing randori in your new hakama? I did several times. What it taught me was to keep my feet closer to the ground when I moved. This may not sound like much of a lesson, but think about how much more unstable you become as your feet get further away from the ground. The other purpose it serves (whether intentional or not) is to hide your footwork. It's harder for Uke to guess/anticipate which direction you're going when they can't see your feet.
Training you to move with the feet lower to the ground is what I consider the most important function though. For that reason alone, I personally think Aikidoka should start wearing a hakama as soon as possible. Rank is rank, and has little to do with the outfit you have on........
two pennies in the pot.
Dan Pokorny - Mongo