I use it as my main machine. If I'm not on my work computer, I'm on my iPad. At home, I only use my tower for hosting media, running backups, and syncing my iPad/iPhone. Once iOS 5 comes out, I imagine I will barely look at my PC, but I will still want it to run media backups and Home Sharing.
I still need my full blown computer, but not to do anything fun or productive on. I guess I'm just not that kind of user. I originally built my current tower to run Crysis. Now it's just a glorified hard drive.