You ask what happens when one omits one of the throughlines. Well, imagine what happens when you lose one of your eyes. You can still see, but the added information you have with two eyes allows you to see things (like depth) that a single eye cannot. Now imagine that you have two sets of eyes. These would correspond to the throughlines: Objective and Relationship Story throughlines are one pair, Main Character and Influence Character throughlines are the other. If you lose any one of them entirely, part of the audiences ability to perceive depth will be removed with it. It is far better to lightly sketch a throughline, than it is to take it out completely.