While anti-virus may not be necessary for the Mac itself, we are after all members of a community. A person may not be offended by his (her) own body odor, but we still use anti-perspirant, colognes and daily showers for the sake of society.
ClamXAV is for me a vital piece of Mac software, if only to be able to clean-up Windows flash drives for colleagues. I use it to scan my mail regularly to avoid passing on the "stink." And, when the first real virus comes along, I'll only have to d/l the newest - free - update. I use it and recommend it - along with a donation to a really good project