RE: xmlns="" ??

Regarding namespace declarations only appearing in the xml at the point
they are needed:

I'm sure this has been mentioned before, but I can't remember what is
the reason is for this?

It seems to make more sense just to declare all the namespaces
referenced in an xml document on the root node - wouldn't this speed up
the processing (elimination of duplicates etc), make it more readable
(as in you don't have to walk back up the tree looking for the
declaration) and be more intuitive?

As I say, I'm sure I've read this before, but if someone could explain
it again for me...

cheers
andrew