A domain controller can do many things besides hosting AD. It can be your DHCP, DNS, Routing and remote services (for VPN and such), terminal server, web server, etc. Most of these can run with a minimal impact on server performance. Unless it is a heavily accessed Web Site. Microsoft does not recommend that you install Exchange on a DC, but I have not had any problems. Of course the sites I maintain all have under a hundred users, so an environment with a lot of users could have problems running on a DC. I am not sure if this answers your question. If not give me a few more specifics on what you are trying to accomplish.