Keeping the laptop out of the network is beneficial for two reasons. One is that my roommate is always using Limewire to download music and other software. He doesn't believe that anything will happen to his system, despite my advice. As such, keeping him out of the DMZ is actually protecting my network.

My second reason is that I don't trust the wireless router. What if someone gets to the router (via port 80 for instance, which allows you to change its settings), or if one of my neighbors is able to crack the wireless password. This is also another mechanism for protecting my network.