Interesting comment!

I like my NAS setup because it's silent (++), easy to manage and it has a really nice Web interface, but you're right your home made server is much more powerfull and flexible and will always be.

Also I tried Plex on the Pi, but it's too slow (there is even a port for the Pi http://rasplex.com/) and it needs the server part of Plex to run on the NAS... at the end it's too much of a burden compared to XBMC for the little Pi.