As Rower_CPU said, Macs *are* vulnerable to Word macro viruses (sort of). The viruses can infect Mac versions of Word (I've seen it happen), and although they can't do any damage, they can infect documents you send to other people, which at best is annoying, at worst can be destructive if they're on Windows.
Turning off scripting will mostly avoid this if you're careful, of course, but if you work with a lot of people using a lot of Office docs, a virus checker might be a decent idea for their benefit.
Personally, I don't use any virus checking software on my Macs, operating on the assumption that if anything starts to spread my daily dose of MacMinute, MacFixit, and here (among other sites) will alert me to it very quickly.
The biggest issue for us are backdoor holes, but that's what firewalls and security patches are for.