Networking :: Firewall As Router - Pros Versus Cons

I'm mentoring my local high school's IT club as they prepare to participate in a cyberdefense competition (see IT Olympics). Generally we are given four boxes and need to set up a network that provides certain services (which services change from year to year, but usually include a web server, email server, FTP server, and an application server of some sort) and support client PCs that connect from the WAN. The red team then tries to break into our network to steal "flags" from our servers and to set their own "flags" on our servers.

Generally we set up the firewall with two network interface cards (one to the WAN and one to our LAN), and connect the LAN NIC to a router, which then connects to the other three boxes. But we do have the option of installing additional NICs in the firewall and configuring it as a router. I can't shake the feeling that there is a security advantage to such a configuration, but I can't say what that advantage is. Perhaps something with configuring ipTables on the internal boxes to accept connections only from the firewall's NIC, and then only for the services we want that box to support (to prevent an intruder from connecting directly from one box to another)?