It is not neccesarry to have it to protect yourself, it helps to protect windows systems if you have windows systems on your network, same with viruses.
You can get PC spyware and viruses but they do nothing but sit on your computer and take up space. the only time i have gotten PC spyware or a pc virus is when i was downloading a p2p program for PC and dint want my PC to get infect with a virus, i kind of use my mac to download PC programs and can scan them on the mac without the risk of erasing my PC.
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