"I guess you don't work as a pro shooter..."
Sometimes I do, with architecture, but the primary format is almost always print. Therefore, if the orientation is not dictated (and it generally isn't), the photo is taken to best show the subject matter. Then, the client wants a slideshow...
Nevertheless, you can say that you always need to consider the intended "output media / format", but that doesn't help, either, with things like an old photo album... 😉
[edit] oh, also - as I said - regardless of whether someone wants to shoot horizonal, vertical, diagonal, or inside-out, iDVD should still be able to rotate images, and should still recognize iPhoto formatting.
Andrew