Old Man's War: Book 3 - The Last Colony by John Scalzi.
I was just as disappointed this time as I was the first time I read it. They introduce a brand new (adapted) technology halfway through the book and then literally nothing comes of it. It helped in the situation they were in, but it should have changed the fate of humanity... and all it was, was a footnote at the end of the story... and it wasn't in the fifth book he wrote, either.
I wonder why the author ignored it.