JSF 2.0 facelets tlddoc says this about cc:insertFacet "required" attribute:
"... If true, and there is no such facet present on the top level component, a TagException must be thrown, containing the Location, the facet name, and a localized descriptive error message. ..."
There is a check on cc:renderFacet, but cc:insertFacet and cc:facet has this attribute too, and we need to implement this description.