Why is xml:base a URI *reference*?

Does anyone here know the reasoning behind this? In specific, why does 
xml:base allow URI references (i.e. with fragment identifiers) rather 
than simply using URIs (URIrefs sans fragment identifiers)?

The reason that I ask is that there are some instances in the RDF and 
OWL PR where xml:base's ending in '#' are used so that the base URI 
matches the XML namespace ... I can see the convenience of this but the 
practice strikes me as a bit weird. Perhaps I am missing something.

Jonathan