I will install anti-virus for Mac for the following reasons:
1. There are known virus, trojan and rogue software for macs. Maybe little types or names of infection for macs but that little amount is expected since the number of mac users compared to Windows is not much and I don't want to be infected in any way.
2. I've seen Mac users reporting infection found in their system so why take the risk? It does not cost much time to run a scan especially if the Mac system has enough memory to run a scan.
3. Apple allow installation of anti-virus program, Apple allow developers to develope anti-virus for Mac. This, to me an evidence that it is OK to have anti-virus.
4. Apple got an old article where they recommended to their customers to install antivirus but they pulled the article calling it old article/support/knowledge base. IMHO, there's no 100% immune operating system. Not even linux, mac or windows. So pulling it does not make Mac not affected at all. Why they don't check with security program developers where they found detection for infections on Mac and maybe Mac will put back the said support/kb.
5. Mac system has their own vulnerabilities.. a good antivirus might help prevent exploit to the 'not yet patch' security flaw in Mac.
6. Mac system allows installation of 3rd party applications like Skype, Firefox, Opera and these products have their own security issue that can "put" users at risk.
7. Mac system is bundled with plug-ins not created by them e.g. java and as we know already, java is another plug-in out there that can be use to exploit, infect or attack.
8. How a Mac user will know if there is NO infection at all, if there's no scanner that will try to determine an infection exist or not?
I can write more but the above is more than enough to me (I hope to other Mac users also to, at least, check if their system or emails is not infected at all), if ever I will own a Mac to use a security tools.
It is also up to the Mac, Linux and Windows users to protect themselves. Not visiting bad sites, not installing rogue products are the same advise that every user, no matter what OS is in use.