That way the carrying capacity of the road is higher. But when traffic is less dense 'station keeping' should be avoided at all times and if someone moves into my 'dead zone' or just to the left of me I'll gradually slow down to force them to finish their overtake.
Why are you in the lefter lane if you can't pass?
I've been in rush hour (where keep-to-the-right-unless-passing is very strictly enforced) in bumper-to-bumper traffic and the left two (out of 6) are completely empty and everyone is doing 'around' the speed limit. Some are in the right lane doing a few below the limit, some are in the left-most lane doing a few above.
Occasionally, someone who is late to work, emergency services, or whatever goes flying by in one of the left-most lanes.
I think it really depends on where/when you're driving. I find this to be a common scenario on interstates during rush hour:
I'm in the right lane, doing approximately the speed limit. There is a safe distance between me and the cars in front and back of me, but only just. If many more cars enter the road, traffic would need to slow down to maintain safe distances. In the left lane is the same situation, except they're averaging about 1 or 2 mph faster. In this situation, there are cars in the left lane passing very slowly, spending a lot of time alongside me. I could slow down below the speed limit every time a car passed on the left, to reduce reduce that loiter time. But this would make my driving less predictable to the drivers behind me (and waste a lot of mileage too...)
So normally, when the other cars are my size, I maintain my present course and speed, driving as predictably as possible to help the other drivers anticipate my course. Changing position in traffic is inherently risky, so I avoid making changes unless doing so is necessary to avoid something I judge to be more dangerous than the average. If a truck passes me on the left, I'll slow down to make the passing faster even if that means a car behind me has to brake. But if in that moment I judge the guy behind me to be even more dangerous, then maybe I won't. It's the kind of decision that needs to be made on the spot in a case-by-case basis. On interstates that are flowing fast near capacity, you need to be constantly evaluating the relative threat of the traffic around you.
Since my time of learning to drive, the requirement to have formal driving training has ping ponged in being a requirement or not. The number of hours as an observer is just as important as the hours being behind the wheel. One of the things repeatedly mentioned by the instructor was to not drive side by side any car unless absolutely necessary. It was also a recurring theme in my repeated defensive driving classes. I also have an uncle that drove trucks for a long time, and he would tell stories of things he saw on the road. A relevant story was when one of the wheels of a tractor-trailor doing 70mph down the highway lost the outside wheel of the trailer and seeing the damage it cause the car driving along side. All of that added together makes me never like to have a car on my sides and I will speed up or slow down (which ever has more space available) to avoid it. For those that did not have to take a driving course, this is just information they may never have been provided.
