I heard a couple of times that hakama gives you a feeling of better balance. It is suposed to concentrate your weight lower so you become more stable. But by the time you get to wear it, you already have to master your "point" and balance. So it helps you do something you already know how to do.
Still - for me at least - it has a certain appeal... I can't wait to have one of my very very own hakamas
Besides, judging from experience, rookies in aikido tend to be fascinated by the sensei's hakama and he gets that much more authority by wearing it. Then again I'm only a rookie myself :/