Depends on how you look at it. Do you know any Windows users that don't have a realtime AV software protecting their computer? I'm of the branch that believe Windows users should maintain their own AV software (as I do - and I still am a regular Windows user - 8+ hrs a day using 4 different Win machines daily) and not have a realtime AV on my Macs.
For years, about once every 9-12 months, it was easier to blow away my wife's computer and reinstall Windows than it was to remove the myriad infections she had on her machine - didn't matter what protection I put on there. I transitioned her over to a Mac mid '07 - she's still using the same Mac - that's gone through 3 upgrades to the OS - I run ClamXav on her machine about once a year - so far, nothing found.
Using a realtime AV is the only way you're going to help protect your Windows using friends if your wife is like mine - forwarding all those junk emails (albeit cute) right along to her friends without a second thought.
Some Mac AV software only protects against Mac malware - some only protect against Windows malware - and some protect against both - time to research prior to just picking one if you decide there is a need. For the latest testing have a read
here.