I wanted to provide some context to "Macs don't need Antivirus" that I see constantly repeated. Now in full discloser I'm an Information Systems Engineer and have spent many years drilling the black and white portions of my brain out, so I don't see things so cut and dry. But in an attempt to provide some prospective I want to help anyone who is asking the question "Does my Mac need antivirus".
So first up lets talk about the software...its a bit disingenuous to call modern security suite software providers just "Anti-Virus". Most of these products provide levels of support for malware detection, adaptive fire-walling, parental controls, and spam filtering beyond just basic anti-virus. So I would suggest that a security suite as a whole works to mitigate multiple levels of potential vulnerabilities in a OS. Are all of these necessary? That very much depends on you as the user and how you use your Mac.
To make a fair determination lets talk use cases. Many different users, use their macs for completely different purposes and at various level of risk:
The easy use case to start with is a Work Mac. I myself have one mac that falls into this category and in this case it is a requirement that I maintain an "Anti-Virus" software. So in this case its not a matter of if, but which. In my case I am allowed to pick, just so long as I can prove that I am using AV of some kind.
Next is the low risk home user...this is where you need to be completely honest with yourself. If you literally check email, browse only legitament non-XXX (or any other kind of potentially seedy) websites and do not use torrents or other P2P sharing services...as in you fly the strait and narrow all the time. Then you probably just fine running without any additional protection. My advice is to do the normal things to secure your mac (Enable firewall, setup a non-admin user account for your day-to-day use, use file vault if you travel a lot) and for a little added comfort download Malwarebytes for mac (https://www.malwarebytes.org/antimalware/mac/) and run it on the odd occasion now and again (Great tool, Only on demand so no resource consumption on your day-to-day use).
High risk home users....you probably know who you are, and are likely (although not always) are more technically savvy. So its up to you, but a little added protection never hurt anyone if your diving deep into the dark corners of the internet. As I mentioned before the security "suite" approach may be more beneficial to you in it additional capacities, not just the AV component (or maybe so if your a mixed OS home). Running Malwarebytes for mac on occasion doesn’t hurt either.
Home users that just want to be as safe as possible....no matter what someone tells you, if removing security software makes your nervous then do what works for you. Trust me, if it gets to be a pain to run any software you will eventually learn to either give it up or move on to another solution. Much of what has been said around "Viruses" and the Mac is very much true, but viruses are only one kind of exploitable avenue to you computer. The OS X operating system is fantastic at security, but no piece of software is ever 100% secure (as the recent publishing of the Gatekeeper bypass shows, http://arstechnica.com/security/2016/01/how-malware-developers-could-bypass-macs -gatekeeper-without-really-trying/, http://www.pcmag.com/article2/0,2817,2497990,00.asp). The point is do what makes your internet experience good for you and if that costs you a good experience with OS X then consider trying other solutions or trimming back to the recommendations I suggested for the low risk users.
As a side note: Blaming AV for high resource consumption is not a 100% relevant argument today (Unless the software is just crap, then its 100% valid). Yes it consumes CPU and memory, but those resources are so large in modern computing that for average users there is no resource contention to conflict with running processes. Yes the real-time scanners do run a 1-for-1 against active process, but the users likely to notice the overall effect will be someone running several active applications or someone like myself who runs several virtual machines at a time.
As to whether or not to use “Anti-Virus”, I think its important to consider the reasons why you want to use it in the first place. Was it for parental controls? Then consider the built-in parental controls in OS X. Firewall....consider the built in? But verify that if you do pick the next solution that it meets your needs as not all security software provide the same capabilities. Also take a look at http://www.av-comparatives.org/mac-security-reviews/ to compare the performance of the possibilities. I have been using ESET for many of years on both the PC and the Mac and still recommend their products, however there are other products (free or paid for) that are also great if you need additional security for your Mac.