There is a slender reason why you should consider anti virus on the Mac - if you receive and forward mail regularly to Windows PCs in a business sensitve enviroment. You can act as an unwitting vector, forwarding infected mail. Essentially a Typhoid Mary.
However I have not seen this happen in the wild.
If youwant top grade and free Anti Virus software for Macs
http://www.markallan.co.uk/clamXav/
All you are wasting is cpu cycles.