This question actually came up during a discussion session at our summer camp a couple of weeks ago. The senior instructors from our organization were in agreement that the reason Hombu changed from having everyone wearing hakama to just yudansha was, as Andrew stated, due to a cloth shortage and economic downturn in Japan immediately after WWII. Therefore, in order to ease the financial burden on aikido students, only yudansha were required to wear the hakama.
Andrew, I'm not clear on your statement about inappropriate sentiments and insults to aikidoka. Was it relating to the previous post?
Anyway, I was recently reading a review on women's dogi jackets by Susan Perry in ATM #64 (Vol. 13, No. 4, July /August '99). In that review she references a book, "Kimono" by Liza Dalby, which states that both the keikio gi and the hakama started out as womens' clothing, and eventually evolved in to being the attire worn for martial arts training. Just thought I'd throw that out.