If you stream from a central computer, you need a (preferably dedicated) central computer that stays on 24/7 365. Who wants to boot a computer each time they watch TV or listen to music?
In a perfect Apple world your ATV would wake up that computer - even wirelessly - through Apple's wake on demand, stream the movie, and put it back to sleep. Pretty cool, but only useful if you're the owner of an imac oder mac mini.