I used to be a PC user since Win95. Used as well Win98, Win98 Plus, Vista, 2000 Professional, XP and the last Windows I used was Win7.
I remember having to reformat because of viruses, thought I used almost all the antiviruses suites, McAfee, Norton and some others I don't remember. Sometimes I used to get a virus that would damage the OS even if it got cleaned by the AV, it removes the virus with some essential windows files, and I end up with saggy and sluggish windows, which would drives me to reformat.
The last time I had a virus was wile using Win7. I was working on Photoshop for essential project when I got hit out of nowhere with a virus that forced me to reformat. It took me 11 hours to restore my machine to the way it used to be before the virus, windows update, with all the photography software I have, plugins, activations bla bla bla .. needless to say I missed my dead line.
At that point I had enough with Windows and its security issues, I decided to move Mac, I bought my first IMac in 2010, and I asked the same exact question, do I need an Antivirus, the agent told me there are currently 0 Mac active viruses, so no need for that, unless you want the peace of mind, but also it comes with a price, as any Antivirus will be scanning network and Internet connection none stop, take some resources and will slow your machine to a degree.
I ended up NOT installing any security software, and in 4 years never had a single problem. Never had a single virus. How do I know that for sure? well I use ''Parallel'' which is a software that allows you to install windows, as my work requires having windows7, and of course I share my work network via VPN to access share drives and resources. I remember couple of times my company got hit by a virus, and my Machine was the only one that didn't get infected, not sure how this happened, but they asked us to run a certain scan software to detect and fix the virus, my entire team was infected, and out of the 12 people, I was the only one who didn't get hit!
Now I have also a MacBook pro, no antivirus and also never a problem.
Something else .. when I used to be a Windows user, I used to get tons of spam. It seems after I moved to Mac, the amount of spam got down from few hundreds a week to maybe 10-20. I know it is not directly related to my OS, but it makes collecting my data harder via adwares and malwares.
Lastly, and not to blow my own horn, I used to work as a computer technician for a long time, I acquired some Microsoft certificates during my service, which were:
MCP: Microsoft Certified Professional.
MCSA: Microsoft Certified System Administrator.
MCSE: Microsoft Certified System Engineer.
The reason I am mentioning these certificates, is to simply demonstrate my PC knowledge, yet I had my own share of problems.
Finally, in 4 years using my Mac OS, I never had a security problem, the performance still amazing, I have to do 0 maintenance to my Machine, no scans, no defragmentation .. nothing, and it still feels like out of the box machine.
My 2 cents,