The attempted argument citing that some branches or groups do not wear the hakama is specious. Some religions incorporate formal prayer rituals and some do not. It doesn't follow that prayer rituals are therefore not an important part of adherence to religious tradition to those who adhere to them, or that the practice is a matter of fashion.
To me, "fashion" connotes an entirely different sort of dynamic: transient, superficial, maybe even somewhat whimsical, and something that is done to garner attention or approval from peers/onlookers. 'Choice' certainly isn't applicable - at least not on an individual level. In most dojo I've run across, wearing a hakama or not is a matter of following the rules and doing things the way your teacher does them. The only element in which there is much of a choice or fashion decision is in whether to get a blue or black one, and whether to get a light or heavy fabric.