At the very least you usually don't want it altered or destroyed. If for no other reason, you need some security just to protect your good name. You wouldn't want some malicious hacker to break into your account and send thousands of hateful messages to every newsgroup in existence. Another reason to secure your system is to prevent it's use as a staging ground for attacking other systems on the network. You could, conceivably, be liable for damages.
Security is a shared responsibility.
Every user on the system is capable of compromising security. They need to chose good passwords, change them periodically, and not share them. Outside hackers are not your biggest security problem. Your highest risks to your data are from bugs and errors in the OS and from disasters. So you need to make sure that you keep good backups.