Does an Active Directory need to have only AD domain controllers?

While it's not a hard-and-fast rule, most AD administrators will tell you that a domain controller should not run other applications. Since your domain controllers are the "keys" to your network "kingdom", you should do your best to isolate them from attack. By adding additional applications to run on a DC, you are increasing the number of ways that a malicious user can attack that DC. Depending on the size of your network, having dedicated domain controllers may also improve performance in terms of user authentication, logon times, etc.