Any discussion should start with a definition of terms. Better people than I have given different definitions, so I'll make it simple. It's your computer, your data, your bandwidth and your resources. Since you put all that money and time into it, it would seem that you would want to hang on to it, yes?
This is how I see computer security. It's awfully similar to security in every day life. You have locks on your home to protect what you have worked hard to gain, so, similarly, you should have "locks" on your computer to electronically protect what's yours.
Well, like I said, you probably want to keep your stuff. Even if you don't, understand that your computer could be used as a launching point for attacks on other sites if it is broken into. The other people on the Internet would like for everyone out their to be neighborly and help cut down on the number of lauch points out there.
Of course, in our increasingly litigeous society, it is just a matter of time until the person in charge of one of the launch points is sued for neglegence or some such. That said, I don't know if such a suit would ever take place, but it's better to be safe than sorry, hmm?
As with most things in life, you have to look at the tradeoff between usability and security. The most secure system is one that is disassembled, ground into a fine dust and shot into the atmosphere. Okay, maybe not -the- most secure, but you have to admit that it would be pretty difficult to get any data off of the system. However, it wouldn't be terribly useful, now would it?
So, you have to strike a balance. It's easy to disable all services on a system, but what if you need one of those services? What if you have to provide a services to others (IE a web server, FTP server, and/or a mail server)? As another example, there have been security holes in the X server in the past. Because of those holes, do you simply not use X? Do you trade off that usability for security? On a personal workstation, maybe. On a Internet accessable server, no, it's not worth the risk.