The Retina MBP has an HDMI and two miniDP, how would those fare when connected to lets say two thunderbolt displays and a 1080p screen and also running de built in display. Those are a LOT of pixels.
All of this on only the iGPU.
I think the dGPU is needed if you are thinking on running external displays.
nope, i can confirm that i can run two 1920x1080p displays as well as the integrated 2880x1800 all at the same time from my base 15" igpu only
i don't have 3 external displays so i can't test that