Hollywood definitely uses lights all day every day for exterior shots, and there's a number of reasons for it...First of all, shiney boards and mirror boards must be adjusted constantly as the sun moves, you can't just set it and forget it, but you constantly have to have a grip shake it up.
The other very big thing is what happens when your sun goes into a patch of clouds? There goes all your light and you've gotta wait until the sun comes back to continue shooting. With lights...if something happens to your sun for extended period of time, it's not extremely difficult to throw up a 4k or 6k HMI to replicate a hard backlight the sun was giving you and then just fill with whatever you've already had up.
I was working on a film quite some time ago where we shot in a forest. There was definitely patches of sun, but since there was so much tree cover, I had to constantly be on the move with this shiney board attempting to find more light to bounce into the scene. Next thing I know, I'm half way up some hill barely getting anything. This would be one situation in which having a few lights and a generator would have been a lot easier, and made things look a lot better in the end. So...I guess the main thing lights get you is just consistency and ease.
rogar-Happy(DBG):