The German laws say "The right lane should be used when it is free and the left lane is generally intended only for overtaking, unless traffic is too dense to justify driving only on the right lane." Look it up.
I totally agree with this law. When traffic is light, it separates drivers traveling different speeds, preventing them from interfering with each other. When traffic is heavy, it makes no sense to clear out an entire lane just for passing, as it would pack the traffic more tightly into the remaining lanes. Plus, it wouldn't be enforceable, because someone could just stay in the left lane and say they're passing the next 100 cars, or that they couldn't find space to change lanes to the right.
Unfortunately, on some crowded U.S. Highways this would mean that the leftmost lane would almost never be available for passing, except during the wee hours of the night.
I often drive home after 11pm, when traffic is light on the freeway, and I try to follow the "move left only for passing" rule then.
I live in Portland
Californians that move up here, have a hard enough time driving in any condition that isn't sunny with blue skies. In rain, they think the far left lane is just another lane to drive slow in. It's not. There is no way to go around these morons, and it causes stressed drivers that take chances. I just saw it today. We need the passing lane rule. If you ever come up to someone driving to slowly, you just pass them, and continue on. Simple, right? (Go home, entitled Californians, we don't want you here!)