Adding an Exchange Server 2003

I currently have two Windows Servers, one is a domain controller and the other is an appserver. I am adding an Exchange 2003 Server. I bought a computer just for this. The IT guy at our parent company wants to use this new computer as the Primary Domain Server with Exchange on it. He is pushing this to our boss. I believe this is a bad idea for many reasons. Anyone have any comments that I can add to my own on why this should not be done? Thanks in Advance.

Steve
Systems Administrator