and suppose something unexpected happens and you are close but have no flash and they need a shot and there is terrible back lighting?
it's happened to me and others and it was interesting to see all the pros asking to borrow the xxD people had for the shot and tossing their 1D's to the side
or maybe you are on a trip and mostly taking landscapes but then want a few shots with friends now and then and don't wanna be dragging a flash along, just one more thing to be a menace and a drag
great if you happen to run around EVERYWHERE, ALL THE TIME, with a reflector
maybe you are coming back at night and suddenly a tree frog pops out?
maybe the Stanley Cup came for a secret visit without prior announcement and they want one pick and everyone is backlit and it has to be taken in that direction and you were not on call but happened to just have your camera with you but no flash and nobody else was close enough to make it in time? I was sure glad I was still using an xxD camera then and, for the record, so was the other guy who showed up and tossed his 1D aside and used my xxD for his shot too. He didn't thumb his nose at the xxD and get all snooty and ridiculous, he was just like awesome, this is great, gotta love the pop-up, all cameras should have this.
I would go back and forth with you all night but I don't think it would get us very far. There are solutions to all of those problems and I don't think the popup flash is the correct one.
When considering light we have to remember 3 main factors; quality, quantity and direction. A popup flash can only address the issue of quantity, which stills leaves poor quality and bad direction. Is that acceptable to you as a photographer? It might well be and that is totally fine but for me (and this doesn't have to apply to anyone else) I find that light to be lacking. YMMV.
Anyway, it's all a pointless discussion. Canon won't include a popup flash on the 5d and 1d series anyway.