To be effective, every tool needs to be used properly and for the appropriate job. A related example is the case of the do-all firewall. A midsize company in New York City was running a network configured as a series of workgroups. No central server, just a hardware firewall appliance that doubles as a NAT and DHCP server for the company's 86 client machines. Then the network crashed.
Virus protection is a perfect example.
While I've seen a few firewalls, both hardware and software, that provide some protection against viruses, this has always been a lackluster effort at best. Right now, there are more viruses and Trojan horse executables floating out there than ever. Using virus protection that was added as an afterthought to another product is just asking for trouble. Normally, I would have recommended a central firewall server that also runs a networked virus checker backed up with personal virus detectors on every workstation. But viruses are only one tier of network security that firewalls don't cover.