Most servers benefit from two NICs; Firewalls ideally have 3. iSCSI is the obvious thing you NEED a second port for, but having separate DMZ and LAN ports is nice even if you have a firewall upstream. We keep server to server traffic for management and rsync on separate networks for fine-grained ups, wake-on-LAN, and SSH logins.
Way to expose your internal network to an attacker coming into a DMZ service. And seriously, the "DMZ" function on most routers is just awful. You should never, ever turn that on for anything. Do port forwardings to a dedicated segment. The firewall should filter external traffic coming in, and traffic going back to the Intranet. Then you have a proper DMZ. Only the select few services you want to expose should be open to the outside.
iSCSI is the only reason to have a dedicated nic. But this is a Mini and it has a FW800 port. Best just connect your external storage to the FW.
The Drobo pro features FW800. This makes a perfect external storage array for this Mac Mini.