dGPU needed for external monitors?

my 13in mbp that only has an igu can run external monitors just fine

 

This has been an "issue" with all of the automatic graphics switching models. They always call the dedicated card whenever an external monitor is plugged in, even if the extra power isn't really necessary. I guess Apple just assumes that if you're plugged into a monitor, you can plug the machine into a power outlet.

http://apple.stackexchange.com/questions/27337/is-a-macbook-pro-not-able-to-use-dual-screen-on-the-integrated-graphics-card
https://discussions.apple.com/thread/5006479

 

The problem is Apple ONLY wired the dGPU to the external video ports. The internal screen is wired to a mux chip that switches the input. That mux chip is connected to both GPUs.
The alternative would be a very complex logicboard with lots more muxing hardware for all the ports. Or Apple could switch to the IMO vastly superior software based graphics switching (like Optimus from Nvidia or Enduro from AMD). In the latter only the integrated GPU is directly connected to the output ports and the dedicated GPU only supplies processing power when needed but lets the integrated handle the display output.

Apple does neither so you have to use the dGPU even if all you do is show still images like slides on a projector for some presentation while you talk.