Same thing I have found in any large setting. THe costs of putting
anti-virus software on over 100 desktops in time, effort, extra
sysadmins, and a dozen other things... makes centralizing it better.
You have to have AV on the clients ANYWAY b/c email is not the only
vector for viruses.
So you're not benefited by centralizing, you're just taking care of it
in two places.
-sv