User-defined Datatypes: owl:DataRange vs rdfs:Datatype

All,

the current OWL 1.1 to RDF Graph mapping indicates that owl:DataRanges 
be used to express user-defined datatypes (such as xsd:int > 18).  Also, 
XSD facets appear to be called owl:<facet>, e.g. owl:minInclusive.

I am wondering why user-defined datatypes are not modeled as instances 
of the RDF Schema class rdfs:Datatype (similar to the hack suggested in 
the Protege 3 implementation [1]).  Without knowing the design decisions 
that lead to the use of owl:DataRange, my naive point of view would be 
that rdfs:Datatypes may make it more consistent with the semantic web 
stack.  I am sure the working group had good reasons for selecting 
owl:DataRange, but it would be useful to understand them from the outside.

Also, I think we should use the xsd namespace for the facet names, so 
that they are written as xsd:minInclusive.

Could anyone please clarify these issues?

Thanks
Holger

PS: The family.owl linked from the OWL 1.1 web site currently appears to 
be inconsistent with the RDF mapping spec (at least with respect to the 
user-defined datatypes).

[1] http://protege.stanford.edu/plugins/owl/xsp.html