The biggest caveat with namespaces is that most people have never bothered figuring out how they work. The number of applications I've seen that have hardcoded namespace names instead of looking up the namespace uri for example, is horrifying.

Namespace prefixes are not that difficult to configure once you know about them. But if you're just starting with XML, probably because you need to extract some information from a document you've been sent, you don't want to learn the theory of XML, you want to get the data you need out and get on with adding business value. So you find a tutorial, you write an xpath, and it doesn't work. You try removing the foo: prefixes in your xpath, and it still doesn't work. This is not the experience that a technology should give new users. A default of matching ignoring namespaces would not make anything impossible.