There's stuff often enough, that it's a good idea to keep your system fully patched. Fortunately, you don't see a lot of stuff like the viruses seen in the Windows world. The vast majority of security-related issues that come up are local exploits that you don't really have to worry about as long as you trust yourself from hacking your own computer.
No operating system is perfect, which is why responsible computer users keep themselves reasonably updated, and keep their attack surface as shallow as it can be.