I know this isn't really the answer you're looking for, but all throughlines are connected by the central inequity. We couldn't get the fullest possible view of that inequity without the RS view.
The MC and IC are both about individuals while the OS and RS are both about relationships. The difference between OS and RS is like the difference between Space and Time--how things are laid out vs how things are played out. The OS stands outside of the individuals and looks at the relationships while the RS stands inside the individuals and looks at the relationships (or something to that effect).
The RS, at least to me, seems to be the one that comes from left field within the quad of perspectives. If OS, MC, and IC provide a more structural view, the RS would provide a more dynamic view. That's why we say the RS is about how the relationship is always growing closer or further, weaker or stronger. That's how the RS functions in relation to the other throughlines.
The storymind justifies (picks an area of the quad in which to attempt to solve the inequity) a personal perspective. This is the MC perspective. But then the storymind will look at the inequity from another perspective to see how the inequity would look from that angle. This second perspective takes place within the same storymind. Looking at the inequity from that second perspective within the same mind will help the storymind to continue building up justifications for its personal perspective, or it will help the storymind to tear down those original justifications so that it can move its personal perspective to that second perspective and justify its new position. Because the very act of looking at that second perspective will reinforce or tear down the storyminds personal justifications, that second perspective is necessarily creating an influence on the personal perspective. This is the IC perspective. The MC and IC perspectives are in the same mind.
When Jim says "the MC is not a real person. The IC is not forcing them to change..." he's not saying the IC is a person who influences an MC person and the way the IC does that is not by taking the MC in a headlock and screaming do it this way but by some other means. He's saying that the MC and IC are both perspectives of, or within, the same person. The influencing of the IC on the MC is just that person deciding which way they prefer to tackle the inequity. The MC and IC characters are just stand ins for the processes of holding each of those perspectives.
The reason that the MC and OS work together and seem so important to the story is, as Jim said, because the current model is built around those two. Look at your story engine. You have MC dynamics and OS plot dynamics. You don't see IC or RS dynamic questions because everything in those is already determined by the MC and OS dynamics. The model was built around the OS and MC because the model itself had to take a perspective.