I definitely don't run Linux for no real purpose. It is my main OS. I use it for gaming, office, browsing, e-mail, irc, etc. etc. I also make a living developing for it. I do agree that it will be an easy way in for hackers though.
I run Linux as my main OS as well... but would you seriously buy a PS3 with the intent to use it as a computer? I certainly wouldn't.
--Zero
If you did use it as a computer, at least you wouldn't have to upgrade the graphics card every year.