Lots of laptops have outputs connected to the nvidia on docking stations, in order to use those outputs you have to really switch to running the nvidia card fully.
(in some situations this also involves copying the panel frontbuffer back to the intel to display). Anything else would just suck. Eventually it might be possible with xinerama improvements to avoid this situation but its non-trivial.
Also valuable Xorg dev time usage is decided by the developers and their employers, and both myself and my employer would quite like to have this project finished already!