You can see both sides really, cant you?
Cars will have to cut in to the remaining lane at some point, all the wagon drivers are doing is moving that point to behind them rather than in front of them. All this does is too make the traffic INFRONT of them to speed up a little as they arnt constantly braking to let people in from the closing lane. Of course, the traffic behind is still affected as it would be if the wagons werent there.
To aid better flow, ALL drivers would have to EITHER:
All move to the remaining lane as soon as possible (ie when the traffic is still pretty free flowing)
OR
All traffic would use BOTH LANES evenly, so the queue in both lanes was equal. Traffic would go into sperate lanes on an alternating basis and then at the point of merge, move into lane in turn.
But, this will never happen of course
Steve