Relative-URI allowed only-if <xml:base> is explicitly provided.

I was reading Tim BL's "Default Base URI for files" 
and got slight flutter.  I was wondering... perhaps 
the notion of "default base URI" is the real source 
of potential inconsistency?  Mabye it would be 
better to forbid relative URIs if the base URI
is not *explicitly* provided.   This might also
provide a better backwards-compatible move towards
relative URIs.

Afterall, Tim's URI RFC explicitly says that
relative URIs should not be used if the 
context is not clear!   If an implementation
has the choice to expand "." as "file:\\\current\path" 
or "ftp:\\localhost\current\path", or any number
of _reasonable_ defaults then I would assert
that the context is not clear.

Best,

Clark