A lot of traffic has no blockage at the front, it's just the compressed bit of a longitudinal wave of cars, which creates a feedback loop of holdups behind it. Having space between cars allows the waves to dissipate more quickly.
In a case of a highway it might not be as beneficial, but imagine a city with lots of road intersections. If you put cars on two lanes and merge, you have half as long line of cars. In case of one lane scenario, the cars might block the previous intersection for other cars.
In a city, you won't have this problems, as traffic will jam at the first road work it encounter anway. Besides, how do ones plan to make all those people collaborate ?
The drivers should know to leave just enough space in front of them for the car on the other lane to merge. Having to stop for a merge makes everything slower. It takes longer to accelerate so longer to clear the jam. Ideally one lane slows down and leaves some spaces between the cars, and the other one merges 1 to 1 without anyone ever having to brake to a full stop.
And I see this working every day since where I live the rule applies. Of course there will always be the occasional smart guy who rides in the ass of the car in front in order to not let anyone pass and save 1-2 seconds at the price of causing everyone behind him 10 times the delay. This forces the cars trying to merge to come to a full stop, then forces some of the cars on the other lane to fully stop in order for the merging cars to be able to accelerate and merge.
No, it's the opposite. In a highway/motorway situation with more queuing space it's not such an issue.
In urban situations where queuing space is limited, using both lanes is more important, as longer tail backs will cause problems with junctions further back on the route.
Sometimes there is an unplanned need to merge, for example if one lane is closed due to construction or an accident (as I think is indicated in the video).
