Michael Kay wrote:

"and they [namespaces] add myriad opportunities for doing things wrong."

You forgot an item in your list: they offer a unique possibility to get things right when things get really complicated.

For example when you do what I presently do: integrate schema information from 283 different schemas. Fortunately they use namespaces - 283 target namespaces - which enables me to keep everything clean without relying on document URIs and or any conventions, and without adding anything to existing structures (like marker attributes). I admit that I miss a lot of fun which I might have if there were no such thing as a target namespace (the shadow of namespaces).