Think of it this way Ian:
In Dreamweaver, without having any webassist extensions installed, there are behaviors for insert, update. delete, user authentication, etc.
These are fairly simplistic server behaviors that where developed by Macromedia in the Dreamweaver Ultra-Dev days. And it was these server behaviors that where the groundwork for the behaviors we created in Data Assist / Security Assist and the other extensions.
The MySQLi server behaviors extension recreates those groundwork server behaviors using the MySQLi library and is the groundwork we will be able to use to migrate Data Bridge and the other extensions to using the MySQLi library.
There where many limitations to those earlier server behaviors, for example; the insert would only support form data. On recreating them, we have made them a bit more flexible.
For now, Data Bridge and the other extensions will still use MySQL. If you find yourself in a server environment that no longer supports the MySQL library, you can use the MySQLi server behaviors to create pages that will work in that server environment.
It doesn't contain the page creating wizards that are in Data Bridge, but does give the basic functionality that Adobe have removed by deprecating the server behaviors.