Tell me agian why I need mml:?

I'm a bit confused as to why I need to qualify the tags in a <math 
xmlns='http://www.w3.org/1998/Math/MathML'/> element.  My reading of the XML 
Namespace recommendation suggests that the default namespace within the 
element should be the MathML namespace, and MathML components should dominate 
identical DocBook components.  Is it simply a case that this isn't a perfect 
world, and current implementations don't behave as they should, or am I 
misunderstanding the XML Recommendations?

Steven