Why we use network-based security devices

This article is a good example of why we use a network-based security scanner.
http://www.theregister.co.uk/2011/02/15/bbc_driveby_download/

Basically, users who simply visit a legitimate website (BBC) could be infected by malware, even if their computers have anti-virus installed. Only nine of the top forty-three anti-virus programs found the malware.  This is why we use not only desktop anti-virus, but also a network security scanner. The network security device is updated continuously, so when such vulnerabilities are detected, they can be blocked, even if they are on a legitimate website.

We are also piloting the use of a "Secure Web Gateway" network appliance to scan all traffic in real time, which would provide another layer of protection.