Allowing URIs as identifier values (in a differently named property) seems
to correspond with the "identifies" property proposed in
< http://www.w3.org/community/schemabibex/wiki/Identifier >: At a first
glance I found it silly that a thing has an identifier which in turn
identifies a (i.e. this?) thing. But if insulate the graph for the
identifier and read "identifies" as "is commonly used for" then it is
exactly the property I was reasoning about.

The important point here is the relationship between the two different resources.  

If both URIs identify the same "thing", in this case a real world person, then IMO the correct predicate is owl:sameAs, and there is no need for a bf:Identifier.

For people, where there isn't any need for interpretation, this is relatively easy.  Either a URI identifies me, or it doesn't.  For more resources that require interpretation, such as bf:Work, or Topic, the same-ness is much trickier to determine as the exact nature of the resource is difficult (I might even say impossible) to determine.  Thus whether an ISBN identifies exactly the same "thing" as another similar identifier is much trickier.

 
Now consider
<http://example.org/persons/kcoyle> a bf:person;
   bf:identifier [
       bf:schema "VIAF";
       bf:identifierValue "195531823";
       bf:identifierValueURI <http://viaf.org/viaf/195531823>
     ].

Or ... <http://example.org/persons/kcoyle> owl:sameAs <http://viaf.org/viaf/195531823> .


This states identity of <http://example.org/persons/kcoyle> with
any resource carrying matching identifieres, e.g.
<http://example.com/p/kc> a bf:person;
   bf:identifier [
       bf:schema "VIAF";
       bf:identifierValueURI <http://viaf.org/viaf/195531823>
     ].

Or ... <http://example.com/p/kc> owl:sameAs <http://viaf.org/viaf/195531823> .

 
However identity with the resource <http://viaf.org/viaf/195531823>
is NOT built into that construction, it's rather "all resources
relating the same way with <http://viaf.org/viaf/195531823>
are pairwise equivalent" and someone would /explicitly/ have to add

<http://viaf.org/viaf/195531823> a bf:person;
   bf:identifier [
       bf:schema "VIAF";
       bf:identifierValueURI <http://viaf.org/viaf/195531823>;
       bf:identifierValueURI <http://viaf.org/195531823>;
       bf:identifierValueURI <http://viaf.org/viaf/sourceID/LC|n89613425>;
     ].

And here is demonstrated the issue with approaches like this...

X a class ;
  bf:identifier Y .
Y a bf:Identifier ;
  bf:identifierValueURI X .

is unnecessary circularity.  Also, having multiple bf:Identifiers, or multiple values on a single blank node like above introduces additional tiers of semantic complexity. 

Rob