It can still be somewhat useful if you're low on clock generators: even an nVidia 1080 only comes with four, many mainstream cards only come with two, and the only alternative if you want more monitors than that involves DisplayPort clusterfuckery. With how cheap smaller monitors are getting, turning on the integrated GPU or throwing an nVidia 710 can be surprisingly useful. Do have to set whatever monitor is attached to the most powerful GPU as the primary monitor. Would recommend against going too many generations back, since both nVidia and ATi's driver installers do weird stuff if you try mixing a 1xxx series and something from back in the GeForce 9600 days.
Do be warned that some motherboards will automatically disable the integrated GPU with a PCIE graphics card installed. Most modern boards will have an option in the BIOS to override it, but some lower-end 'business' boards don't.