I wonder how much of it is just marketing, opposed to actual 'need' of anti-virus software...
Even when I was using Windows, I was very light on the anti-virus software. I never had more than one AV program installed, and sometimes didn't have anything. I've never had a virus on my computer.
Since I've been running Linux, I haven't used any AV software at all. That's been maybe the last 6 years, something like that anyway.
There have been cases where the supposed "anti-virus" software was, itself, malware...
I can't imagine having to run more than one program for a specific task, let alone 9!