That's where you build a database agnostic DAL and your actual database layer is written to support the RDBMS of choice. To provide a system that performs well on both Oracle and SQL Server, for instance, you should have to separate code bases at the database layer. This allows you to code each to make the best use of the RDBMS features available.

Extra cost, probably. Better performance for the application on both systems, probably. If a customer were to change the RDBMS would you be in a better position to retain the customer, (again) probably.