Like
Inzil, I've read this chapter many times before and I would have been quite happy if Tolkien had written many more like it; I am one of those readers he talks about in his letters as wanting "more information" about everything. In this respect, I almost like the two appendices here more than the main text itself. The schema of Rohan's military hierarchy and a consideration of the geopolitical importance of Isengard, Helm's Deep, and the Fords of Isen together are just the sort of information that I would love to have more of--and that I am glad I am able to have at all.
I am perhaps the reverse of
Esty in that I never really wondered why the Battles of the Fords of Isen were not part of the main LotR text. I took it in the manner of which it was written--which is to say peripheral--as indicative of the actual importance of the battles. It took reading this chapter of
Unfinished Tales to change that position. At the same time, however, I don't really think it needs to be in the main LotR text. After all,
The Lord of the Rings is a story following the main characters (Hobbits and Aragorn, to limit them to the smallest possible number), NOT an epic covering the entire War of the Ring. Speaking of epics, there's good precedent for only showing a slice of the war--and not even the most important slice.
The Iliad, after all, shows only 2 weeks of a 10 year war and does not include the beginning, the end, or even the death of Achilles.
Nor is this the only battle of the war left out. What about the Battles in Lórien, Mirkwood, Erebor, or Bree? We only hear of these after the fact, and not always in great detail. Yet the Battle of Erebor, if no other, was at least as crucial as the Pelennor. Similarly, the first salvos of the war, when Sauron forced the passage of the river, don't even get a detailed description in "The Hunt for the Ring" where they're a relatively greater plot-point than in the LotR-at-large. Like all of these battles, the Fords of Isen (I & II) are left out because they don't involve our main characters--or even our secondary characters after they've been adopted into the main narrative (like Éomer circa "The King of the Golden Hall" or Imrahil around "The Siege of Gondor."
All of which is to spend, perhaps, more words on the matter than it necessarily warranted... but the CbC could use a good tussle, right?