I get that many XML documents don't use namespaces or schemas. I also understand that you can have an XML document that use a namespace without any associated schema (e.g. Log4J configuration). 

While it is technically possible to create an XML schema without an associated namespace, isn't it the case that nearly every XML schema has it's own unique target namespace?

Maybe there are some that constrain multiple namespaces, but I can't think of any examples of that either.

Follow-on question: If you were to version your XML schema (and its URI), would you version your namespace URI?