Not sure my opinion counts as it's been almost 2 years since I seriously looked into forum admin, and I just peek in here from time to time because I like the cozier feel of old-school forums compared to modern social media.
Anyway, I don't see the harm in allowing users to create their own communities assuming the feature is off by default. My only issue, and I'm not sure it's even an issue, is that this, along with NodeBB's use of ActivityPub, pushes NodeBB into the niche of other Threadiverse platforms like Lemmy and Piefed. The only difference is the terms used and default presentation. Don't get me wrong the defaults have a HUGE impact on community culture. As I've pointed out elsewhere something as simple as the default sorting order of posts effects how people browse, post, and organize the content they present. Lemmy is, by default, a stream of new content, whereas forums are, by default, a place for long-lasting discussions.
Allowing user-created subforums would, I suspect, have a similarly huge effect on how NodeBB communities are run by mods and admins and how they feel to users.