The default behavior of the SiteIndex.pm content handler is to include all page layouts into the sitemap.xml output. Some users may not want non-navigable pages to be included in the index. On the other hand, it may be helpful to show all the layouts in a site whether they appear in a navigation or not.

I think a switch in the conf file would allow the user to select the setting they'd prefer. Personally, I think the default should be to hide pages that aren't available for navigation (as does Colin).