When including (typically using <ui:include> tags) a xhtml page (lets say A) into another xhtml page (lets say B), the navigation page A.page.xml is not processed. I could not find a statement on that behavior in Seam documentation, but I added logging in the org.jboss.seam.navigation.Page class to highlight that point.
Having a A.page.xml page in the project is misleading: the developper may think that it is processed by Seam while it's not. And moreover, this may introduce side effect when migrating to a future version of Seam that could take this A.page.xml into account.

So when a page is included, Seam validator should issue a warning/error saying "The navigation file .page.xml file of included xhtml pages is not processed by Seam. You should consider removing it" if a corresponding page.xml file exists.