According to Markus' email:
In the end, all RDF properties should have a declaration. This is a single triple declaring the rdf:type as owl:ObjectProperty or owl:DatatypeProperty. The domain and range specifications are an optional add-on that mainly serves as a documentation for the ontology.
So, we may want to add definitions and rdf:type classes to property predicates (such as wdt:P22 and p:P22) when generating property data (for dump or when generating mentioned entities for Special:EntityData).