Yea not sure why he is. I guess when you are just web browsing it could potentially put more stress on it, but it wouldn't be anything really, just a small blip. It would still be at full idle almost always.
As for gaming, if you play any modern games during which you don't maintain 60FPS constantly, your GPU is being stressed as hard as it possibly can. Adding an extra monitor will make your FPS go down more, but it doesn't have the ability to add more stress because the card is already working at its full ability.