[Morthoron]

Lord of the Rings is paced as it is because it is Hobbitcentric. In other words, if it did not touch the four main Hobbit characters directly, the extraneous storylines were told as if second-hand information. Tolkien maintains this formula for most of the books, except in the case of 'the hunters' Aragorn, Gimli and Legolas, who warranted inclusion because they are members of the Fellowship. Therefore, the battle scenes you alluded to do not adhere to the formula Tolkien set, and would detract from the aspects the author wished to draw the reader to.

I think we all wish for aspects of the story to be expanded. Things like Galadriel laying bare the pits of Dol-Guldur, or the Battle of Erebor, both of which only managed a few sentences of text, but which could have obviously been expanded into full-blown tales in and of themselves. Unfortunately, time and temperment were more enemies than allies to Tolkien, and he was a putterer and a perfectionist. He himself was not satisfied with many aspects of his creation. He just didn't have time to do everything he wished to do. Who does?