Can anyone tell me why this isn't an inbuilt feature of OS X? Preferably in the 'Energy Saver' pane of System Preferences, e.g. while on battery power, only use integrated graphics.

There was a discussion that sometimes it is better to run a task quick on GPU than to run it for longer time on CPU. Would be cool if someone could try to prove this or inverse. :)

Maybe it'll be different in Metal-backed El Capitan, but my experience so far has been that the discrete GPU universally degrades my battery life. I'm glad I have it though; sometimes I need it, but it'd be nice if I could just disable it completely most of the time.