QName element texts and attribute values except for schema types can not be uniquely identified with an URI
It is wrong to assume, that the default namespace applies to all QName strings, that are element texts or attribute values.
In fact exisiting prefixes can not be uniqely resolved without application logic and no prefix does not imply any namespace either, not even the default namespace.
For example this applies to port type equality checks.
Edited by Maximilian Pilz