I think it depends on how close to 100% you can guarantee that your windows clients are virus free. Depending on permissions on your shares and files, all it could take is one infected windows machine in your entire network to infect thousands of files on your filer. We've had this happen probably at least a half-dozen times. We just have way too many windows machines in our network: desktop machines, laptops that people take home and hook up to their home networks, and often the worst of all lab machines since we often can't fully control how those machines are set up. We have probably hundreds of machines running IIS for web development, trying to keep all of them patched is so far an extremely difficult job. You can block viruses through your mail gateway, but are you blocking employees from accessing hotmail or msn or any other web-based mail? Because viruses can easily sneak into your network from there. Are you blocking/filtering ftp? Because that's another way. Not to mention the various P2P file sharing services.
We have Norton corporate edition deployed throughout most of our desktop clients, which in theory should be keeping all the clients up-to-date, but we find far too often that the automated client updates aren't happening, or that the client AV software is not running for some reason, etc. From what I've heard from people using other vendors like Symantec and Mcafee in big enterprise environments, they can be plagued with similar problems as well.
We're still working on deploying the Netapp AV solution from Trend (budgeting and planning can sometime take *way* too long) but when we deployed a trial version we were very impressed by the efficiency with which it stopped virus infections onto the file server. We started getting notices of attempted virus infections within minutes of activating the software.
For a long time when people complained about viruses on the file server, I would take the position that if we just prevented virus infections on the client side there would be no problem. But I've come to accept that it is just virtually impossible to guarantee that you can keep every client on your network up to date and virus-free at all times. And manually scanning the file server regularly just isn't feasible, given a very large file server it can be like painting the golden gate bridge, and with today's viruses it can only take a window of a few minutes to infect thousands of files.
So, in summary, I'd say that virus protection on the file server is very important, unless you are *very* confident that your windows clients will always be virus free, or perhaps if your file server is very very restrictive permission-wise, such that an individual infected user can't do anything more than damage their own personal folders, *and* you have good frequent backups.