How can I demonstrate the importance of OS-level firewall?

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http://serverfault.com – I've come across some Windows administrators that are disabling Windows Firewall because "it makes troubleshooting more complicated". The systems are protected by firewall appliances at the edge. However, I have not had a successful time articulating the importance of enabling Windows Firewall---or any OS-level firewall for that matter. I was hoping someone in SF might have some information that might assist me: Some information from an OS developer (Windows or non-Windows) with regards to the importance of the OS-level firewall A demonstration or example of an exploit that is only possibl (HowTos)