Reply #21: Your last sentence is the ENTIRE point.

[A HERETIC I AM]

My take is that the left lane is a passing, and if you are not passing, regardless of the traffic conditions, you ought not be driving in it

Absolutely spot on. It isn't about whether or not a driver is going the posted speed limit, it is about lane discipline and traffic flow. The safest speed to travel, regardless of the posted speed limit is that speed at which 75% of the traffic is traveling at. Having said that, the left hand lane is the passing lane and once the passing maneuver has been completed, one should move back to the right as soon as is safely possible.