Firewalls and AVs are like condoms. If you careful and make sure that everyone you sleep with are trusted and clean then it's unlikely that you'll get an STD. But if you're promiscuous and sleep with anyone you find then don't be surprised if it burns while you pee.
I've had an AV installed for years, but I've not actually had anything detected for years either. Do I need it? Probably not, but it's that one time when that cute girl looks respectable but is actually rather slutty that you need to be weary of, in which case it's always prudent to be protected.
Burnsy
Nice analogy

