Why does MacbookPro Retina require the dedicated GPU to access external displays?
It's likely that the external display connections are simply wired directly to the discrete GPU. The internal display is connected to a gmux chip that handles switching between GPUs and it probably would have been significantly more complex to route the 3 external ports the same way.

I think it's fair to assume that most users connecting external displays will have AC power available, so this seems like a fine way to simplify things.

It's definitely not an issue of graphical power - I use an external 2560x1440 monitor with the new Iris Pro-only 15" rMBP and previously used it with an Intel HD 3000-equipped 13" MBP, both of which handle it fine.