Uh, except you don't? Maybe I'm reading your post wrong, but you don't need a PC or Mac to use AppleTV. I use mine all the time without a computer on... and I haven't opened iTunes in weeks. But I use ATV almost everyday.
No, you're just missing my earlier posts in this thread.
If you have a library that is too big for one ATV, or if you have multiple iTunes clients (PCs/Macs, ATVs, AirPort Expresses) then you need a centralized repository. Unfortunately Apple so far have required that repository to be a full fat computer.
To put it another way, sure the ATV can store a (small) library of music and video, but without a running Mac, what use is an AirPort Express? You can stream to the Express from the ATV of course, but that's the problem: the ATV is not up to the job of being a server.
A Mac Mini is overkill. A slightly beefier AEBS that can run a stripped down iTunes as a server would be ideal. It'd be a few hundred bucks cheaper than a Mini, use less power than a Mini, and would bring its own wireless access point.
It wouldn't do anything you can't currently do with your Mac, but it'd:
a) allow you to turn the Mac off.
b) allow you to take the Mac out of the house (laptop).
c) allow you mac to do whatever it is it's supposed to be doing without having to find IO bandwidth, and CPU cycles to power media terminals around your house.