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.
TV which can act as the central host for my iTunes library so I can maintain it seamlessly across multiple computers.