I use a cable and u-lock combo when I commute in the big city in order to secure the frame and both wheels.

I use a light cable lock when I tour. Not going to carry a heavy u-lock, and it would often be useless in a campground and pointless when there’s hardly anyone (or no one) around.

Contrary to popular belief, there aren’t always people lurking behind every tree and lamp post just waiting to steal your bike.