The MySQLi server behaviors extension is a starting point for upgrading the other extensions to use MySQLi.
Think of it this way, the current extensions where built on the groundwork of the standard Dreamweaver server behaviors. We needed to recreate that groundwork using MySQLi before we could start upgrading the extensions.
At this time, eCart 6 has been upgraded for MySQli, and are working on Data Bridge, we are targeting the First Quarter of 2015 for the Data Bridge upgrade, and then will be working on Design Extender.