... just as a more flexible model would favour people who prefer that mode of working.
I guess it boils down to what kind of culture you want in your organisation. I‘m involved in a national students‘ association where we have some very strict structures and are highly organised in the way we work. A close partner association of ours is much more free-wheeling, embracing individual freedom and spontaneity over formal processes.
In my experience, our structure helps to give us continuity in the face of constant high member turn-over (students are usually only around a few semesters). This gives us stability, and ideally means that we can concentrate on developing content rather than organisational details. The other association rarely manages to conduct projects on the scale we often work on. In that sense, our structure is highly effective.
On the other hand, we sometimes get lost in the process - ideas die in committee, that sort of thing. Because we don‘t want to move fast and break things, we sometimes struggle to move at all. And like you say, not everybody enjoys working in such a structured environment - those students tend to join the other association. To be fair, they often have more things going on than us, in part because they aren‘t scared of just giving it a go and seeing where it takes them. Their flexibility can be a big strength as well as a weakness.
So it depends on what you want, and I think that in many cases, people join (or stay at) organisations whose style of working suits their own structur-flexibility preferences.