Virus protection on servers?



I have 13 Windows based servers. Many of these were installed over the
last 3 months as we made the transition from being primarly a Novell
shop to Windows based servers. Here is the breakdown:

2 domain controllers/dns/dhcp
2 Exchange 2007
ISA Server
SQL Server
IIS
6 Application Servers

What is the consensus on whether I should have virus protection
installed on these servers? I do not have any file servers since my
NetApp SAN takes care of that. I do own McAfee's Enterprise package
which includes McAfee's VirusScan plus their add on Spam protection
piece. We have this product installed on all of our client machines, I
am not sure if it is a good idea to install on servers.
.