I hate to be a wet blanket, but there are virus writers out there who are after the bigger challenge of Mac viruses. In other word, backed up or not, you're at risk. And even if all they do is wipe your hard drive, are you willing to risk the time and effort it takes to restore your system when they hose you?

It's ont thing to not be too worried about how much damage an intruder can do to your files. It's another to allow such intruders to use your system to serve their files to their cronies. I would be personally terrified that some wiseguy might plant something in an unprotectd system that would allow him to serve up illegal files. He might, just for fun, plant kiddie porn on such a machine, then call the FBI. Not worth it for me! To me, an unprotect system is an invitation for some aggressive intruder to mess me over, so I don't leave those doors open.