No, it's because the other Coalition members don't trust Szeth, for obvious reasons, why on earth would he fear Skybreakers?
Well, I would.
I agree it was poorly handled, and several of his chapters could have been cut as well, especially the Dog and the Dragon.
And how do they tie into the main plot?
I would have agreed with you, if Venli had chosen to stay with the Coalition forces, but going back to the Shattered plains was pretty much her exiting the story. I highly doubt they will play a role in book 5, and will probably only get Interluedes where we can see them.