The quote shows what the people in the castle thought of capping, and also it is when Will realises he does not belong there, in the castle. That brings the whole idea of what he left for, which was freedom from the tripods. What Will did was leaving to find freedom and end up in a place like the place he just left. He had totally forgotten about his freedom thing, it seems, until he thought this quote.
The chapter is relating to the whole freedom thing in that is the chapter in Will realises he does not belong. He ran away and found himself in the same situation as before. That is also shows that he would probably leave anyway, and prove again that freedom thing. Also, he was happy at first, and then he found out Eloise was capped and wanted to leave. You could tell he was torn- between freedom and the person he loves. Will made a hard choice, but Henry and Beanpole would have gotten there slower than if they were together. So that was all part of the story.