Metamodeling seems to me (and has proven to be) convenient in Wikidata. Think that there is a lot of classes, and that regrouping classes with some similarity in metaclasses can have a lot of applications, inlcuding:
* tag classes in different ontology with some ''instance of'' <class of this ontology> metaclass to help query the class by intology
* regroups class of the same kinds : classes of chemical elements can be regrouped because they all class atom with respect to their atomic numbers. This is what we call a chemical element and is consistent with the definition of chemical element in the corresponding french Wikipedia article.
* regroup classes by definition of their instances : for example the laws of a country actually defined implicitely a lot of classes. Those classes can be regrouped using metaclasses like <class defined by french law>
* ...