Isn't that what XML did - you only put the namespace in if it was needed. As for multiple ones in the same object it makes sense if you want to reuse a definit

Isn't that what XML did - you only put the namespace in if it was needed. As for multiple ones in the same object it makes sense if you want to reuse a definition used elsewhere e.g. to add an address using a predefined address type. It is like using structures/records in programming languages but with no pointers for composition.