is the clock illuminated or is just the LCD that stays on? if its just an lcd, it has the tiniest power drain ever. Think of those huge LCD clocks at sharper image or some other crappy store, that run w/ AAA batteries. My guess it's tiny.
It doesn't make sense why more cars don't do it? Why should they?
It makes reasonable sense why they don't, because it's unnecessary for the average schmoe. Unless a burglar needs to know how long he's got to jack your stereo, or you walking to your car; opening the door and look at the dash to see the time and back out.