1) A database stores data. It does not care where that data comes from.
2) A database returns data. It does not care where that data goes.
3) A database manages data. It does not manage emails and user interaction.
4) A database outputs data. It does not care whether that data is used for an invoice or for a report.
5) A database holds tables. It does not care whether or not the tables completely define a customer.
6) A database is a database. Joumla, and any other applications, are responsible for integrating with it.
If it's not practically useful, then it's practically useless.