Christopher Davis commented:
It would make it impossible for non Foundation members to create rooms on the homeserver, which would limit a lot its adoption.
Alternatively, since applications are migrated manually to the World/ or GNOME/ group, it could make sense to create the room on the homeserver at that moment if the maintainer asks one, and grant them Moderator access on the Matrix ID of their choice.
The federated nature of Matrix rooms makes this a non-issue. Admins would not need to create a new room at all when a developer gets an LDAP account. Instead, the developer can join their existing room, set their power level appropriately, then add an "alias" to the room and publish it to the room directory.
You can start a room e.g. on #myroom:matrix.org, and then add a #myroom:gnome.org alias once you get an LDAP account and publish it to the directory.
This is what Fractal, Sound Recorder, Podcasts, and other primarily Matrix chat rooms have done.