Well as far as electric power savings go it sounds funny to me, but the idea can actualy be very usefull for other reasons such as overpopulation of certain areas!
There is an issue of uldahs main area being overcrowded mainly by douches that leave their chars in active mode with best gear equiped just to show off, and go afk for days... Forced log out after few hours of idling would certainly improve the situation imho. Less afk epeen douchbags = less lag for the active ppl.