A Mac Mini is aleady a NAS drive

I just noticed this news snippet Turn your Mac mini into a NAS drive. It's a box that stacks nicely with a Mac Mini and works as NAS storage. I think the principle of NAS storage is great, but surely if you already have a Mac Mini it would be easier (and cheaper) just to add an external USB drive and then share that? Mac OS X includes file sharing support, for Mac, Windows, Unix (NFS) and FTP - the NAS only supports Samba (Windows) and FTP. A separate NAS box I can understand if you didn't want it near an existing computer, but if you are going to place the two together, then making use of what you already have would make more sense. Certainly this unit isn't cheap - there's no drive included in the price, and for only a little more than the cost of the bare NAS box I just bought a 500GB USB drive.