In our dojo we start to wear the Hakama from sankyu and up.
As a 7th kyu I dont get to wear one, and although I would really like to, I can see why I shouldn't. My instructors can evaluate my posture and footwork more easily and it is a great incentive for me to train and be able to wear that some day.
Regarding the question of importannce of the hakama, I strongly believe that it is an important part of the aikido training. For me it symbolises the 7 virtues, and reminds me that I am studying a civilised and noble art.
Just my two cents.