It's actually quite hard to read the The Disposition of Names in an XML Namespace above the sound of thumping tubs in the background, and I was rather worried that the balance had been struck rather too finely between evolution and intelligent design until I hit:

As a general rule, resources on the web can and do change. In the absence of an explicit statement, one cannot infer that a namespace is immutable.

Huzzah! So to state the bleeding obvious, namespaces are all about ownership. They're a land-grab. I always put spuds in your weekly vegetable box, but don't send it back if I happen to slip you the odd free carrot as well. Feel free to use or ignore them!