The motors only require the GPS lock to start (when it's not disabled). However, that's only to originally start the motors. Once they're started, you're fine.
If you're flying and you lose GPS lock, the quad doesn't just crash out. It instantly beeps at you, flashes on the controller, and goes to full manual mode. That means that you have no stability from the GPS, so you are now at the completely whim of the elements. It's not the same as angle mode, because now you have to control every aspect and adjust for everything, every breeze, and so forth. So while indoors in say, an auditorium, you won't have wind blowing you around, you DO have to fight that outside if you lose GPS lock.
Also, outside, it WILL still keep trying to get a lock, but until it re-establishes, you're manual.