Not all documents have a base URI.

What happens when you POST an XML document? There is no URI for the client...

What happens in a pipeline when you chain two XSLT stylesheets together? The first result has no URI...

This means that documents can be broken after you author them.

This link to 'bio.xml' has no resolveable URI under HTTP POST:

<person bio="bio.xml">
<name>Alex Milowski</name>
</person>