Until SQL Server 2012, it wasn't an option to backup file groups, so containing staging tables in a separate database was the only way to prevent backups from including a lot of unnecessary and volitile data. Even today, the case can be made for containing the staging tables, not just on separate database, but on a separate instance. Actually, you may even want to contain your staging environment outside SQL Server. For on-premisis staging databases, I've entertained the idea of installing Linux/MySQL on left over hardware.