IWhen Ueshiba himself explains what aikido is all about, he doesn't say anything about clothes, buildings, or rituals -- at least as far as I have read.
As far as I know he spoke about wearing hakama? Wasn't he also strict about rituals?
But, maybe he did not emphasize those things because a lot of it was just self-evident for him?
I sometimes have made the experience that the behaviour of Europeans in a d˘j˘ can be a real culture-shock for Japanese teachers. I remember End˘ sensei lecturing at length about the correct use of zori when leaving the tatami about three years ago.
On the other hand I experience Japanese to be much more uncomplicated regarding rules of correct behaviour and clothing. But I think this is because they dont't have to think about it. They behave intuively and know the fine line between "free" and "wrong".
Regarding the question of this thread:
Throughout our whole life we wear different clothes matching different occasions, situations or places. And we behave corresponding to these settings.
So I think wearing hakama, observing proper etiquette and other things are important as far as practicing in a d˘j˘ is concerned. The setting of the d˘j˘ needs those things. And they also teach something in this situation.
But aikid˘ is not only practiced in a d˘j˘. One of the most important lessons I ever got from my teacher happenend in pub near a d˘j˘. No etiquette, no dogi, nothing of that.
Finally, I think there are certain aspects constituting aikid˘ that have to be practiced in the formal setting of a d˘j˘. If you would take them away you would loose parts of the core of what at least I belive aikid˘ to be.