Dave Beckett in A retrospective on the development of the RDF/XML Revised Syntax:
A new syntax should be closely based on the RDF graph via the terminology in RDF Concepts and Abstract Syntax so that it is complete, and also take into account the requirements given earlier (Section 6). In particular the critical requirements (Section 6.1) will be met if it closely aligns with the abstract syntax.
A new XML syntax that looks like the abstract syntax will tend to seem like an XML-ized version of N-Triples, if it is minimal. This is sufficient but does not meet the additional XML requirements (Section 6.2) that suggest using some more modern XML design ideas e.g. QNames. At present RDF/XML uses QNames only as the element and attribute names however newer XML work such as W3C XML Schema use and allow them as attribute values to identify concepts that are identified by a (namespace name, local name) pair. RDF does not use such identifiers, so QNames could only be make to define or refer to URI-references, blank node identifiers or literals. This suggests continuing the RDF/XML approach of concatenating the (namespace name, local name) to give a URI. However, QNames used in this fashion cannot encode all URI-references so cannot be used as the sole way to encode identifiers for RDF graphs, and thus there must be a way to give any URI. This tends to suggest having either both QName-style and longer URI-style approaches. However, allowing QNames in element content (or attribute values) causes problems such as invisibility from XML processors, XML Namespace scoping and with XML Canonicalization. Mixing QNames with URIs in similar fields can cause interoperability problems since the syntax of both are very similar - ex:prop is a syntactically legal QName and URI with URI scheme ex.
Using QNames in attribute values is evil, please let's not go down that route.