Just to throw my 2 cents in:
There are Linux virii, but they are less common. The most recent is acutally a crossplatform virus:
http://www.symantec.com/avcenter/venc/data/linux.simile.htmlVirus protection on the desktop is important, but it doesn't replace the need for a server antivirus. You will always have a user who sets up their own machine, brings in a outside machine, or uninstalls their virus program. A server solution also protects you from incomming virii, taking the end user out of the mix before they can panic at a virus pop warning.
Having protection on the server gives you one place to update and be sure that your network is protected. I find that to be a lot easier than running around to 10 desktops and making sure that all the hotfixes and virus updates have been run as soon as a new virus is out.
Having said that, always run your network the way you see fit. If you don't see a need for server side protection, don't get any. It probably won't be a problem. But in the end, how much is that "probably' worth?