Lets not get confused here..
There is an Active Directory Agent for BE12 and higher. It allows you to backup a DC, and restore individual objects like users, groups, OU's, etc... Without having to go into AD recovery mode, and no reboots when restoring users.
Without the specific functionality in 12 or 12.5, you are just installing a standard windows agent to backup a server that is a DC. When you need to restore AD, you are restoring the entire System State and all of AD. Plus a couple of reboots in between.
You will need an agent installed on the DC, to back up it's System State, and Active Directory information
Technically the Remote Agent for Windows (RAWS) is all that is needed to be installed on a host you want to backup. It's the licenses entered in the media server that will dictate whether or not you can use the application specific functionality for Exchange, SQL, Active Directory, etc...