Its probably very worthwhile checking out AV-Comparatives - a fairly respected neutral organization that checks out a variety of OS antivirus programs on a regular basis - and its Mac Security Review. If you want to live in a closed environment and do nothing but trade iWorks documents between other Mac users, swell - but otherwise, the bragging rights to Mac OS being the best AV is sketchy - and really, if it can be done for free with a minimum of fuss, what's the problem? Then there's emails and documents sent from other OSs that may unintentionally have something embedded that might not affect me on my Mac but cause a misery if forwarded along - so yeah, I use an AV program because I can and its no stress. Hey, even Apple HQ got hit with malware earlier this year.
