Really? I picked up xmlns when trying to figure out Microsoft's .NET1.1 XML serializer. In addition to your object's schema, it imports a few extra namespaces for the schema definition language (xsd) and schema instance (xsi) features... in particular you need a schema instance attribute "xsi:type" to resolve type names for polymorphic things.
Used like that, it feels just like importing a library in a programming language - I have a reference up top on the root element saying "this schema has this namespace and this prefix" and then I have attributes and tags with that prefix peppered through my document in addition to my own object schema tags and attributes. I can see how a lot of xmlns features could be really opaque (I don't understand any of them) but in this use case it was pretty straightforward.
Used like that, it feels just like importing a library in a programming language - I have a reference up top on the root element saying "this schema has this namespace and this prefix" and then I have attributes and tags with that prefix peppered through my document in addition to my own object schema tags and attributes. I can see how a lot of xmlns features could be really opaque (I don't understand any of them) but in this use case it was pretty straightforward.