There are definately use cases for different databases which are NOT covered by just using different notebooks.

Let's say, we have

  1. Clearly delimitable information collections (eg. work/home)
  2. Some devices on which we need access to ALL collections (eg. the personal PC)
  3. Other devices on which we want to have access to ONE collection, but NOT the others (home vs. work mobiles, shared PCs at work vs. at home)

There are good reasons to disallow one or more collections on devices of type 3: data security policies, sync efficiency, memory limits on the device, ...

Joplin's current approach to enable only one information collection makes it impossible to synchronise all collections with devices of type 2.