"Looks like you ran into
it may be related however symptoms are not the same: it's not that "IF none of the poms (module or parent) don't have an url tag,
THEN site inheritence quitely isn't applied, for example in the parent's site.xml".
what I observe is : the child's site is not built at all if the url tag is missing. and an exception is raised (exception.txt).
a difficulty for reporting these issues correctly is that behaviour of inheritance seems quite instable. Sometimes, menus are not inherited, sometimes, only menus from parents are shown. Sometimes, skins are not inherited, or partially, sometimes they are...
In the example bug2.zip, with the site plugin checked out from trunk and rebuilt yesterday,if I run "mvn site" in the parent's folders, menu "menu1" is not inherited.
And I'm sorry for the numerous small posts of yesterday : I tried to add a comment to each uploaded file, but clearly it does not work like that !