I'm confused about the usage (or purpose) of REASSIGN.
Reassign ownership to make it easy to drop a role.
If I've understood the docs correctly, REASSIGN changes the ownership of all database owned by old_role to new_role, as well as all objects on the *currently* connected database.
Correct.
It seems to me that the most common task for an administrator would be to either:
change ownership of all objects on a single database owned by old_role without affecting any other databases or change ownership of all objects on all databases owned by old_role.
As I understand it, I can't use REASSIGN to perform either of these tasks. Or am I missing something?
Nope, that's it. You need to execute REASSIGN on each database to change ownership of all objects on all databases.