Sometimes I think Apple just jumps out of a moving train. For example it isn't possible to turn off the notebook display and only keep the connected monitor/projector active. Or, if you wake it with the lid closed to turn on the (notebook) display at a later time without disconnecting the external display.

Same with this. I can understand why they do the switching the way they do it, but where is the harm in putting a "Don't use discrete graphics" option in the Energy-Saver menubar drop-down menu? If they believe their customers are that stupid display a dialog box warning them, with the option to never show it again.