I think that the ability to have two different wordpress databases could help for at least two scenarios:
- Easy migrate contents from different environments (development, test, preproduction, production), especially during a first development-release process. This would allow to export all wordpress system tables between each environment, leaving content's tables untouched
- Better separation of the most important data (contents) with the ability to keep them in different servers
Of course this would be an optional feature, as no everyone can have more than one database.
But I wonder, since this has been never done, maybe I'm missing something?