The main reason for having two NICs would be so you could have your camera network be separate from the rest of your network. The cameras would not have access to the internet or to any other computer on your network besides the BI PC and the other cameras, so you would be safe from practically any security hole in the camera firmwares. You would not even need a router for the separate camera network as long as you assigned static IP addresses to everything. You might even be able to configure Windows to act as a router for the camera network if you wanted, but I would not bother.