I dislike people who drive below the limit in the left hand lane - in high school, I came that close to putting a paintball gun behind the grill of my car to deal with them; the problem with getting paintballs filled with the right stuff to eat off their paint was the show stopper. (In hindsight, that was probably a good thing for me.)
On the other hand, in some places - notably major roads in and around Austin, TX (including I-35) - keeping vehicles out of the left hand lane EXCEPT to pass would effectively cut the highway's traffic capacity by a third to a half.