No, we don't need it.

1) As macskeeball stated, there are no know viruses in the wild.

2) I don't care if I do pass on a Windows virus. If they get infected, it's their own damn fault for using a Windows PC.

I don't mean to sound heartless here, but I'm not about to start running virus software on my Mac just so I can help PCr's avoid viruses.

If and/or when a virus is made that'll affect OS X, I'll start using something like ClamXav.