I'm of the opinion that ALL games that have NPCs need the PC-NPC-PC triangles to have those NPCs be interesting.

But what are the implications of choosing lower classes? That doesn't seem to be spelled out mechanically for the different classes other than the general implications of the Rank mechanic.

It seems like taking a lower rank signifies that you want to have fewer obligations, but you pay for that with less social power and (more importantly) less control over the shape of the world.  The people who choose the higher rank options will make more choices about how the game world is set up, and thus have more story control.