[sciurius]

I'll add a use case.

I have songs for several different projects (band, choir, etc.). The songs are distinct and I use collections to group them.
Within a collection, I need another level of grouping. There are songs ready for use, songs that need practice, songs that are under consideration, and so on. I currently use setlists for this, but this tend to get confusing with the 'real' setlists.
When working on a project, I want all project-related songs at hand and I'd prefer none of the other songs to show up. They just get in the way.
As far as I can see, this would best be realised with separate databases, one for each project. Within the project I can use collections for the song categories, and setlists for setlists.
If there's a good alternative I'd be interested to learn.

Separate databases would also be helpful in setting up small example sets for testing, without the risk of damaging real song collections.