I couldn't disagree more, the Horse and his Boy and it's characters fit seamlessly within the Narnia narrative without having to introduce an entire new cast of characters like, say The Magicians Nephew does with a thin tangential thread. We already know the Pevenseies at this point who are involved in the plot, which is actually a fairly compact story when one removes descriptions of food. It's also necessary to get to know that there are real people living real lives in Archenland when it is ultimately destroyed by Aslan in The Last Battle, otherwise without these senseless deaths how will we learn about the majesty of Aslan's mercy, or whatever?