O'Brien's direct, all-business tone has a lot to do with keeping the reader glued to the story. This isn't one of those stories where you are doing a lot of guessing or trying to read the author's mind. And the narrators are not trying to get you so emotionally wrapped up in these characters that you can hardly turn the page for fear of crying.
Instead, the book is all about making the story move along quickly. Does it work? We sure think it does. Take as an example the moment when Mrs. Frisby is trapped in the birdcage, desperate to get out and warn the rats:
The day after next the truck would come with its poison gas and that would be the end of all their plans. Unless they could be warned. Wearily, she got up to climb the wall and try again. (25.11)
We don't have a lot of emotional language here. "Wearily" is about as touchy-feely as it gets. Instead, it's just the facts: a lady in a birdcage with a need to get out. So brief, and yet it tells us everything we need to know. Plus, because the story fits together so well, we already know that Mrs. Frisby wants out of that cage—we know she has information that the rats need and we know that she feels a responsibility to help others. There's no need to get all melodramatic on us when we already know what's at stake.
That's not to say, though, that the tone doesn't have a flair for the dramatic every once in a while:
The weight of the world seemed to fall on Mrs. Frisby's tiny shoulders as she realized that she was trapped. Trapped! Captive! A prisoner! Now how would she tell those brave, smart, selfless rats that their death could be driving this way any minute? It was all too much for Mrs. Frisby, who saw her children's small, sweet faces flash before her eyes before she sank down, down, down to the bottom of the birdcage in a deep faint.
These moments of melodrama help us remember that, while we're reading a story about a bunch of rodents, those rodents have very real dreams, hopes, fears, and feelings—just like us. If it were all facts, all the time, this story probably wouldn't hit home as powerfully as it does.