I guess.
Would you be happy infecting a friend's windows computer with an infected PDF file or similar?
For the seven years I have used Macs, one month or so was with an AV scanner, Sophos.
Never have I got a reply from someone I send an attachment to, that they got infected by something I send. While it is nice to protect others, and I am a sucker for that, the person running one's own computer is also responsible for protecting his or her own computer.
The point about the hacking was brought up by others about Sophos, but popular software used on Mac presents a current threat. If Sophos or any other Mac Anti-malware product can protect mac users against things such as Mac Defender is that not a good thing? There are times we all underestimate the blind faith shown by humans, don't assume that others are as savvy as you obviously are. As I've said, I have Sophos on Macs for the other family members, I don't believe in people wanting to give me millions or that paypal really needs my details.
I know, that I am more knowledgable than the crowd using Macs, I see it almost every day, but if someone asks here for advice about AV software for Mac OS X, I just recommend reading that Malware FAQ
GGJstudios kindly put together. If that person gets more educated doing so, the better. If the person still wants to use an AV software of some kind, because s/he doesn't understand it or doesn't want to live without AV software, then that is fine too.
But if someone is educated enough and understands the concept of common sense and malware and what to avoid, then why should that person use precious CPU cycles for AV software?
Again, in the seven years I used Mac OS X, I never came upon malware, and I even visited all the shady sites I can't think of, except hacking and coding sites, and am still malware free. I will continue to do so until I might encounter some kind of malware, but until then I am glad not having AV software in my computer. I just don't like the interference.
PS: I am not afraid of being hacked, as I am behind two firewalls, which of course can be hacked by some knowledgeable person, but I am not really a good target anyway. If I would have sensitive data on my Mac, I might have to employ other security tactics. But my current tactics fared me well to this day, so why use AV software?