When they do this in during congestion it slows traffic more and also causes "steals" spaces from the traffic that is already on the highway or freeway.  The way it should work is you go to the end of the merging lane and then alternate with the people on the highway letting one person in ahead of them.  When someone jumps in before the end of the lane then they will be letting an additional car in ahead of them and everyone behind loses an extra couple of car lengths.