Why does MacbookPro Retina require the dedicated GPU to access external displays?

Question

This is a general graphics question although I would especially like to apply it to my Early 2013 Macbook Pro Retina 15". I have gfxCardStatus running and whenever I connect one of my 1280x1024 or 1920x1200 external monitors, it tells me it has to switch to the dedicated graphics chip to continue running - and promptly the battery indicator in the top menu bar recalculates a significantly lower remaining time (when running on battery). Here is another example where connecting multiple external screens made the fans in the Macbook Pro go mad and the whole system react slowly even when otherwise idle.

This confuses me for two reasons.

1. My old Macbook Pro (2009, non-retina, internal GFX) could effortlessly drive the same external display with only an internal graphics chip. Why can the new Macbook Pro not do that?
2. I have been using multiple monitors since 2000 when I bought my first TFT and hooked it up to the second output of my NVIDIA Geforce2 card. Why do graphics cards in 2013 choke at driving three or four screens when graphics cards in 2000 could do two easily (even with 3D support) and hardware nowadays is several orders of magnitude faster?

Answer 1

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.