It's like drivers with the right of way who stop to let someone out of a driveway, when they have a line of cars behind them.

The only situation in which that is appropriate is if there is a red light ahead with a line of cars between you and the red light. That way there is no real loss to stopping and letting someone in, because you and everyone behind you were going to have to stop in 30 feet anyway.

Unfortunately most drivers are retarded and think that stopping one lane of two lane traffic is going to help someone trying to make a left turn.