When is it appropriate database design to use a separate schema?
I am going to mention both logical and physical design here.

I know IBM recommends placing staging tables into their own schema(s) and even into their own table spaces (database implementation specific to DB2 in this case) for reasons of management. Schemas allow you to manage better via separation of concerns and security. The tablespaces (physical design) allow you to better handle maintenance issues regarding REORGs, RUNSTATS, and backups.

While you may not be implementing IBM DB2, I think their philosophy is something worth thinking about and implementing.