Who really needs a buildin flash? There are a lot of small flashunits out there (f.e. 270EX II), you can stick on the
body of you want. And those are far superior to the buildin-lightbulbs.
You know the quote "The best camera is the one you have with you?" Applies to flashes, too. With the built-in flash, that "Ooh, there's not quite enough light" moment takes a single button press to fix. Without it, you're digging in your bag and fishing out the flash, then swearing when you realize you have to dig again for a replacement set of batteries, and two minutes later, you realize you missed the shot two minutes earlier.
Yes, I often know I'm going to need the flash, and put it on ahead of time. But I'd be lying if I said I always did.
With an external flash, I also have to choose between having a flash and having my radio remote, because there's only one shoe to hold them both. With the built-in flash, it only takes a quarter extra second to slide the remote receiver out of the way so that the flash will flip up.
Finally, even the 270 EX II is huge compared with the built-in flash. I realize that this helps in terms of avoiding a lens shadow on the floor, but it's a considerable amount of weight and size.